ci: nightly master tarball workflow

[RvVeen]

Summary

Add a GitHub Action that builds and publishes a plugin tarball to a rolling
_master prerelease on every push to master, so users can install the latest
build directly from a stable URL without waiting for an npm release.

Changes

• .github/workflows/master-tarball.yml — on push to master (and manual
  dispatch): bun install, bun test, bun run typecheck, bun run build,
  then npm pack and upload the tarball as opencode-kiro-auth.tgz to the
  _master prerelease, keeping the tag pointed at HEAD.
• .github/workflows/release.yml — bump actions/checkout from v4 to v5.
• .gitignore — ignore packed *.tgz tarballs.

Usage

Install the nightly build in opencode.json:

{
  "plugin": [
    "https://github.com/<owner>/opencode-kiro-auth/releases/download/_master/opencode-kiro-auth.tgz"
  ]
}

Testing

• Workflow runs the full bun test + typecheck + build gate before packing,
  so a broken build never publishes a tarball.

[tickernelz]

Merged into master as 2bd36a30ca1df00bb34de7ea7f97b882c2890efe. The combined newest-to-oldest integration stack passed TypeScript typecheck, build, Prettier, and the full Bun suite (140 passed, 0 failed). The new master tarball workflow also completed successfully: https://github.com/tickernelz/opencode-kiro-auth/actions/runs/29880354693