chore: migrate package management from yarn to pnpm

[JeevanMahesha]

Description

Migrates the root workspace off Yarn and onto pnpm for consistency with the docs/ subproject, which was already on pnpm. Specifically:

• Replaced yarn.lock with pnpm-lock.yaml (via pnpm import to preserve resolved versions)
• Added a root pnpm-workspace.yaml (packages: - 'projects/*'), replacing the workspaces field in package.json which pnpm doesn't read
• Added onlyBuiltDependencies to pnpm-workspace.yaml so native postinstall scripts (esbuild, @parcel/watcher, lmdb, etc.) aren't silently skipped
• Updated package.json: added packageManager/pnpm engine fields, swapped internal yarn ... script calls for pnpm ...
• Added .npmrc with shamefully-hoist=true — required, not cosmetic: without it, the built dist/spectator/jest and dist/spectator/vitest entry points fail to resolve @testing-library/dom and @jest/globals types, since pnpm's strict linking doesn't hoist transitive deps to root node_modules the way Yarn classic did
• Updated .husky/commit-msg and .husky/pre-commit to call pnpm
• Updated .github/workflows/ci.yml: added pnpm/action-setup, switched the cache key to pnpm-lock.yaml, swapped all yarn run commands for pnpm
• Updated README.md install instructions and .gitignore

docs/ is untouched — it already had its own pnpm-lock.yaml/pnpm-workspace.yaml and remains a fully independent pnpm project; deploy-docs.yml needed no changes.

Related issues

Fixes #

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes the public API)
• Documentation only
• Refactor, test, or chore (no user-facing change)

Breaking changes

None — public API and published package are unaffected. Local contributors must have pnpm installed (corepack enable or npm i -g pnpm) and should delete their old node_modules before running pnpm install.

Test plan

• pnpm build — full ng-packagr build across all entry points (@openng/spectator, internals, jest, vitest), schematics build/copy, docs copy; verified dist/spectator contains expected output
• pnpm test:ci — Karma/Jasmine (326/326), Jest (242/242), Vitest (212/212) all passing
• pnpm lint — 0 errors (pre-existing unrelated warnings only)
• pnpm test:types — passes against built dist output
• Verified .husky/pre-commit and .husky/commit-msg execute correctly under pnpm
• Confirmed docs/ subproject is fully untouched (git diff --stat -- docs empty)

Checklist

• Issue discussed or bug clearly described (link issue when applicable)
• Tests added or updated for behavioral changes
• Documentation updated (README, JSDoc, migration notes as needed)
• Public API changes documented; breaking changes called out
• CHANGELOG updated (if the repository maintains one and the change is user-facing)
• Commit messages follow Conventional Commits
• I agree to follow the OpenNG Foundation Code of Conduct

Additional context

Contributors will need to switch from yarn to pnpm locally (pnpm@10.33.1, pinned via packageManager). No dependency versions changed — this is a tooling-only migration.