Add release/package evidence mapping for SourceOS product packaging surfaces

[Copilot]

This PR makes homebrew-tap the distribution evidence surface for SourceOS packaging while keeping runtime/policy authority in source repos. It adds explicit release operation coverage, artifact evidence records, and traceability links across all required product surfaces.

• Release evidence model (operation-plane compatible)

  • Added release-evidence/workspace-operations.json with:
    • required operation types: release.package.prepare, release.formula.update, release.checksum.verify, release.evidence.attach, release.rollback.record
    • integration target links
    • per-surface release evidence records for:
      • SourceOS-Linux/BearBrowser
      • SourceOS-Linux/agent-machine
      • SourceOS-Linux/TurtleTerm
      • SourceOS-Linux/sourceos-devtools
      • SourceOS-Linux/sourceos-syncd
  • Each record includes package formula, checksum record, release note, rollback note, build/test evidence link, source commit reference, and provenance reference.

• Packaging surfaces updated to carry evidence links

  • Updated caveats in existing package definitions:
    • Formula/agent-machine.rb
    • Formula/bearbrowser.rb
    • Casks/bearbrowser.rb
  • Caveats now expose release evidence/checksum/rollback references directly at install time.

• Missing product surfaces bootstrapped

  • Added formula scaffolds:
    • Formula/turtleterm.rb
    • Formula/sourceos-devtools.rb
    • Formula/sourceos-syncd.rb
  • These provide packaging entry points and evidence linkage without introducing runtime governance behavior in this repo.

• Governance boundary documented

  • Added README.md describing evidence mapping and the hard rule: packaging/distribution here must not bypass source-repo contracts, tests, or policy gates.

# Example pattern used in formulas
RELEASE_EVIDENCE_RECORD = "https://github.com/SourceOS-Linux/homebrew-tap/blob/main/release-evidence/workspace-operations.json"

def caveats
  <<~EOS
    Release package evidence:
      #{RELEASE_EVIDENCE_RECORD}
    Checksum record:
      #{RELEASE_EVIDENCE_RECORD}
    Rollback note:
      #{RELEASE_EVIDENCE_RECORD}
  EOS
end