feat: GitHub Actions workflow to build & package MV3 extension for Chrome/Edge stores

[Copilot]

Adds a CI/CD workflow that packages the Chromium MV3 extension into store-ready ZIP artifacts on every PR, push to main, and v* tags. Also bumps manifest.json version as part of the release baseline.

Workflow: .github/workflows/build.yml

• Triggers: pull_request, push → main, tags: v*
• Package manager detection: lockfile-based priority (pnpm → yarn → npm → none); installs + caches deps accordingly via Corepack where needed; gracefully skips when no package.json exists
• Build detection: scans package.json scripts for build, build:extension, build:prod, dist, bundle in order; skips silently if none found (pre-built extension case)
• Extension dir detection: dist/ → build/ → extension/ → src/ → repo root → find-based fallback; fails with a clear error if manifest.json is nowhere found
• MV3 sanity check: fails fast via jq if manifest_version ≠ 3 or name/version fields are absent
• Versioning:
  • Tag builds (v1.2.3): syncs manifest.json "version" to the tag before packaging
  • Non-tag builds: validates existing version matches X.Y.Z semver; fails with error if not
• ZIP packaging: files staged in a mktemp dir via rsync (excludes .git/, .github/, *.zip, *.md, LICENSE), then zipped with cd "$STAGE" && zip -r . — guarantees manifest.json at ZIP root, no nested folders, no nested ZIPs
• Artifacts uploaded: chrome-extension.zip, edge-extension.zip, artifact-info.txt (commit SHA, branch/tag, build cmd, ext dir, pkg manager)
• GitHub Release: created automatically on tag builds via gh release create using GITHUB_TOKEN; contents: write scoped to job level only

Version bump

File	Before	After
manifest.json → "version"	"1.1.0"	"1.2.0"

No package.json exists in this repo — manifest.json is the sole version source.

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