ci: add release workflow to publish extension zip on tag

[rrbe]

What

Adds a GitHub Actions Release workflow so GitHub releases are produced automatically, instead of by hand.

Learned from the termdown project's release.yml and adapted for this pnpm-based browser extension:

• Trigger: push of a v* tag (or manual workflow_dispatch).
• Permissions: contents: write so the action can create releases.
• Version guard: fails fast if the tag (v1.8.1 → 1.8.1) doesn't match both package.json and static/manifest.json — borrowed from termdown's "verify tag matches Cargo.toml" step.
• Build & package: runs the existing pnpm run pack script (build.mjs → dist/ → zip), then names the asset better-github-v<version>.zip.
• Release: softprops/action-gh-release@v2 with generate_release_notes: true, attaching the zip.

Single-artifact extension, so no multi-target build matrix (unlike termdown's cross-platform binaries).

How it's used

Future releases: bump the version, tag vX.Y.Z, push the tag — the workflow builds and publishes the release with the packaged extension zip attached.

Note: the v1.8.1 tag was pushed before this workflow existed, so it won't have triggered a release. After merge, re-running the workflow via workflow_dispatch on the v1.8.1 tag (or re-pushing the tag) will produce that release.

🤖 Generated with Claude Code