chore: add linter

.cspell.json

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+{
+  "version": "0.2",
+  "language": "en",
+  "caseSensitive": true,
+  "useGitignore": true,
+  "ignorePaths": [
+    ".github/**",
+    ".cspell/**",
+    ".gemini/**",
+    ".vscode/**",
+    ".cspell.json",
+    "**/*.svg",
+    ".gitignore",
+    "package.json"
+  ],
+  "dictionaryDefinitions": [
+    {
+        "name": "custom-words",
+        "path": ".cspell/custom-words.txt",
+        "addWords": true
+    }
+  ],
+  "dictionaries": [
+    "custom-words",
+    "aws",
+    "bash-words",
+    "companies",
+    "css",
+    "data-science-models",
+    "data-science",
+    "data-science-tools",
+    "acronyms",
+    "shared-additional-words",
+    "en_GB",
+    "en_US",
+    "filetypes",
+    "fonts",
+    "fullstack",
+    "go",
+    "google",
+    "html",
+    "java",
+    "k8s",
+    "mnemonics",
+    "monkeyc_keywords",
+    "node",
+    "npm",
+    "people-names",
+    "python",
+    "python-common",
+    "shell-all-words",
+    "softwareTerms",
+    "webServices",
+    "common-terms",
+    "sql",
+    "tsql",
+    "terraform",
+    "typescript"
+  ]
+}

.cspell/custom-words.txt

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+# cspell-specific custom words related to UCP
+Adyen
+Alam
+Amex
+Ant
+Anytown
+Backordered
+Braintree
+Carrefour
+Centricity
+Chewy
+Commerce
+Credentialless
+Depot
+EWALLET
+Etsy
+Flipkart
+Gap
+GitHub
+Google
+Gpay
+Kroger
+Lowe's
+Macy's
+Mastercard
+Paymentech
+Paypal
+Preorders
+Queensway
+Sephora
+Shopify
+Shopee
+Stripe
+Target
+UCP
+Ulta
+Visa
+Wayfair
+Worldpay
+Zalando
+adyen
+agentic
+atok
+backorder
+checkout
+credentialless
+credentialization
+datamodel
+dpan
+ewallet
+fontawesome
+fpan
+fulfillable
+gpay
+ingestions
+inlinehilite
+linenums
+llmstxt
+mastercard
+mkdocs
+mtok
+openapi
+openrpc
+paypal
+permissionless
+preorders
+proto
+protobuf
+pymdownx
+renderable
+repudiable
+schemas
+sdjwt
+shopify
+superfences
+vulnz
+yaml
+yml

.github/ISSUE_TEMPLATE/bug-report.yml

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+name: 🐞 Bug Report
+description: File a bug report
+title: "[Bug]: "
+type: "Bug"
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for stopping by to let us know something could be better!
+  - type: textarea
+    id: what-happened
+    attributes:
+      label: What happened?
+      description: Also tell us what you expected to happen and how to reproduce the issue.
+      placeholder: Tell us what you see!
+      value: "A bug happened!"
+    validations:
+      required: true
+  - type: textarea
+    id: logs
+    attributes:
+      label: Relevant log output
+      description: Please copy and paste any relevant log output. This will be automatically formatted into code, so no need for backticks.
+      render: shell
+  - type: checkboxes
+    id: terms
+    attributes:
+      label: Code of Conduct
+      description: By submitting this issue, you agree to follow our Code of Conduct
+      options:
+        - label: I agree to follow this project's Code of Conduct
+          required: true

.github/ISSUE_TEMPLATE/feature-request.yml

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+name: 💡 Feature Request
+description: Suggest an idea for this repository
+title: "[Feat]: "
+type: "Feature"
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for stopping by to let us know something could be better!
+  - type: textarea
+    id: problem
+    attributes:
+      label: Is your feature request related to a problem? Please describe.
+      description: A clear and concise description of what the problem is.
+      placeholder: Ex. I'm always frustrated when [...]
+  - type: textarea
+    id: describe
+    attributes:
+      label: Describe the solution you'd like
+      description: A clear and concise description of what you want to happen.
+    validations:
+      required: true
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Describe alternatives you've considered
+      description: A clear and concise description of any alternative solutions or features you've considered.
+  - type: textarea
+    id: context
+    attributes:
+      label: Additional context
+      description: Add any other context or screenshots about the feature request here.
+  - type: checkboxes
+    id: terms
+    attributes:
+      label: Code of Conduct
+      description: By submitting this issue, you agree to follow our Code of Conduct
+      options:
+        - label: I agree to follow this project's Code of Conduct
+          required: true

.github/linters/.markdownlint.json

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+{
+    "default": true,
+    "MD013": false,
+    "MD007": {
+        "indent": 4
+    },
+    "MD033": false,
+    "MD046": false,
+    "MD024": false
+}

.github/linters/.yamllint.yml

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+rules:
+  line-length:
+    max: 80
+    level: warning

.github/pull_request_template.md

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+<!--
+   Copyright 2026 UCP Authors
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+-->
+
+# Description
+
+Please include a summary of the changes and the related issue. Please also
+include relevant motivation and context.
+
+## Type of change
+
+Please delete options that are not relevant.
+
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] **Breaking change** (fix or feature that would cause existing
+      functionality to not work as expected, **including removal of schema files
+      or fields**)
+- [ ] Documentation update
+
+---
+
+### Is this a Breaking Change or Removal?
+
+If you checked "Breaking change" above, or if you are removing **any** schema
+files or fields:
+
+- [ ] **I have added `!` to my PR title** (e.g., `feat!: remove field`).
+- [ ] **I have added justification below.**
+
+## Breaking Changes / Removal Justification
+
+(Please provide a detailed technical and strategic rationale here for why this
+breaking change or removal is necessary.)
+
+---
+
+## Checklist
+
+- [ ] My code follows the style guidelines of this project
+- [ ] I have performed a self-review of my own code
+- [ ] I have commented my code, particularly in hard-to-understand areas
+- [ ] I have made corresponding changes to the documentation
+- [ ] My changes generate no new warnings
+- [ ] I have added tests that prove my fix is effective or that my feature works
+- [ ] New and existing unit tests pass locally with my changes
+- [ ] Any dependent changes have been merged and published in downstream modules
+
+---
+
+### Pull Request Title
+
+This repository enforces **Conventional Commits**. Your PR title must follow
+this format: `type: description` or `type!: description` for breaking changes.
+
+**Types:**
+
+- `feat`: A new feature
+- `fix`: A bug fix
+- `docs`: Documentation only changes
+- `style`: Changes that do not affect the meaning of the code (white-space,
+  formatting, etc)
+- `refactor`: A code change that neither fixes a bug nor adds a feature
+- `perf`: A code change that improves performance
+- `test`: Adding missing tests or correcting existing tests
+- `chore`: Changes to the build process or auxiliary tools and libraries
+
+**Breaking Changes:**
+
+If your change is a breaking change (e.g., removing a field or file), you
+**must** add `!` before the colon in your title:
+`type!: description`
+
+**Examples:**
+
+- `feat: add new payment gateway`
+- `fix: resolve crash on checkout`
+- `docs: update setup guide`
+- `feat!: remove deprecated buyer field from checkout`

.github/workflows/conventional-commits.yml

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+#   Copyright 2026 Google LLC
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
+name: "Conventional Commits"
+
+on:
+  pull_request:
+    types:
+      - opened
+      - edited
+      - synchronize
+
+permissions:
+  contents: read
+
+jobs:
+  main:
+    permissions:
+      pull-requests: read
+      statuses: write
+    name: Validate PR Title
+    runs-on: ubuntu-latest
+    steps:
+      - name: semantic-pull-request
+        uses: amannn/action-semantic-pull-request@v6
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          validateSingleCommit: false