ci: automate releases with release-please

[wyne]

Summary

• add release-please configuration seeded from the shipped 0.1.7 release
• let release-please maintain CHANGELOG.md and the source version in main.go
• upload the fasder-X.Y.Z.tar.gz source archive to each GitHub release for the Homebrew tap
• add a conventional PR title check so future release notes and version bumps are classified correctly
• keep release.sh as a manual fallback that archives the requested tag

Setup note

RELEASE_PLEASE_TOKEN is not currently configured on wyne/fasder. Add a fine-grained token with contents, pull request, and issues write access if you want release-please PRs to trigger normal CI checks. The workflow falls back to GITHUB_TOKEN, but bot-created PR commits may not run the usual checks.

Validation

• actionlint .github/workflows/release.yml .github/workflows/pr-checks.yml
• ruby -rjson -e 'JSON.parse(File.read("release-please-config.json")); JSON.parse(File.read(".release-please-manifest.json")); puts "JSON OK"'
• GOCACHE=/Users/justin/workspace/fasder/.gocache go run -buildvcs=false . --version
• GOCACHE=/Users/justin/workspace/fasder/.gocache go test -buildvcs=false ./...
• GOCACHE=/Users/justin/workspace/fasder/.gocache ./release.sh 0.1.7
• regenerated fasder-0.1.7.tar.gz checksum matched the published release asset: afa2bba06ff7fb1b2b83ec3cb087d974eb0e30d309ca24ec9635b60a0f0d57a2
• git diff --check