refactor: move from npm to pnpm

[joshblack]

Update the project to use pnpm instead of npm. This allows for improved dependency resolution in the monorepo (namely around hoisting) and install performance locally and in CI.

Changelog

New

Changed

• Update workflows to use pnpm
• Update package.json and scripts to use pnpm
• Add missing dependencies in various package.json files to support correct node_modules layout

Removed

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

This is a change to the package manager of the project only

[changeset-bot]

⚠️ No Changeset found

Latest commit: 35495ef

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[joshblack]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Conflicts are resolved and merged with latest main in 76bddca (with a small follow-up workflow text fix in 379abc5).

[joshblack]

@copilot fix CI

[Copilot]

@copilot fix CI

Fixed in 35495ef by regenerating pnpm-lock.yaml to match merged package manifests. This resolves the pnpm install --frozen-lockfile CI failures.