chore: add CI workflows and migrate to bun

[peter-svensson]

Summary

• Add CI workflows matching other sparetimecoders messaging repos
• Migrate from npm+vitest to bun (bun:test, bun install, bun publish)
• Add Codecov coverage reporting
• Add release-please automation with bun publish to GitHub Packages
• Add review gate, auto-merge, Renovate, and semantic PR config

Workflows added

• ci.yml — typecheck, test with coverage, build
• release.yml — release-please + bun publish
• review-gate.yaml — PR review enforcement
• enable_pr_automerge.yaml — auto-merge with squash
• renovate.json + semantic.yml — dependency updates and PR title validation

Test plan

• CI passes (typecheck, test, build)
• Coverage uploads to Codecov
• Renovate picks up the config
• Semantic PR check works

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️