docs: add PR submission guide

docs/how-to-submit-a-pr.md

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+# How to submit a PR
+
+## 1. Prerequisites
+
+- Make sure you have write access to the repository (or submit a PR from a fork).
+- Use Node.js locally (recommended to match `package.json` `engines`).
+
+## 2. Update `main` and create a branch
+
+- Pull the latest `main`.
+- Create a new branch (recommended naming):
+  - `feature/<short-desc>`
+  - `fix/<short-desc>`
+  - `chore/<short-desc>`
+
+## 3. Development and local checks
+
+- Make your changes.
+- Run minimal local checks:
+  - `npm ci`
+  - `npm run prepare`
+  - Optional: run a quick manual validation relevant to your change.
+
+## 4. Commit conventions
+
+- Recommended commit message prefixes:
+  - `feat: ...`
+  - `fix: ...`
+  - `refactor: ...`
+  - `chore: ...`
+
+- Keep the PR focused: ideally one PR addresses one category of change.
+
+## 5. Push your branch and create a PR (recommended: gh)
+
+- Push your branch:
+  - `git push -u origin <your-branch>`
+
+- Create the PR:
+  - `gh pr create -R nmhjklnm/cue-command`
+
+- Suggested PR description:
+  - Why (motivation)
+  - What (changes)
+  - How to test
+  - Risks / impact
+
+## 6. CI and review
+
+- Ensure all PR checks pass (CI runs on PRs).
+- Address review feedback by pushing additional commits.
+
+## 7. Merge policy
+
+- This repository uses a ruleset: only `squash` / `rebase` merges are allowed.
+- Generally, **squash merge** is recommended to keep `main` history clean.
+
+## 8. Post-merge
+
+- After merging, confirm `main` CI runs and succeeds.
+- If a release is required, follow the repository release process.
+
+## 9. Common pitfalls
+
+- **CI did not run**
+  - Confirm your PR targets `main`, and check workflow triggers (`pull_request` / `push` branch filters).
+  - Use `gh pr checks <PR_NUMBER> -R nmhjklnm/cue-command` to see if checks are present.
+
+- **CI fails (works locally but fails in CI)**
+  - Prefer `npm ci` (depends on the lockfile) and keep `package-lock.json` in sync with `package.json`.
+  - Use `gh run view <RUN_ID> -R nmhjklnm/cue-command` to inspect the failing step.
+
+- **Merge button disabled / merge method rejected**
+  - The ruleset allows only `squash` / `rebase`; merge commits are not allowed.
+  - Selecting a disallowed merge method in the UI will be blocked by the rules.
+
+- **Conflicts / need to sync with `main`**
+  - Update your branch with the latest `main` (rebase or merge per team preference).
+  - After resolving conflicts, push again and CI will rerun.
+
+- **Useful gh commands**
+  - View PR: `gh pr view <PR_NUMBER> -R nmhjklnm/cue-command`
+  - View diff: `gh pr diff <PR_NUMBER> -R nmhjklnm/cue-command`
+  - View checks: `gh pr checks <PR_NUMBER> -R nmhjklnm/cue-command`
+  - List main runs: `gh run list -R nmhjklnm/cue-command --branch main`