ProofFlow v0.1.6.1 - Dogfood Polish for CI Review Comments

ProofFlow v0.1.6.1 - Dogfood Polish for CI Review Comments

This patch comes directly from dogfooding the v0.1.6 GitHub Actions review
workflow on ProofFlow itself.

The workflow worked: it ran on a real pull request, published a stable summary
comment, uploaded summary.json, and exported a downloadable Proof Packet.
The first real review also exposed a small usability issue: the PR comment
showed the runner-local Proof Packet path, which is technically accurate but
not helpful to a maintainer reading GitHub.

Fixed

• The PR review comment now tells maintainers that the Proof Packet is included
  in the proofflow-agentguard-review workflow artifact instead of displaying
  a /home/runner/... path.
• The stable marker update behavior was verified: a second push edited the
  existing ProofFlow comment instead of adding a duplicate comment.

Docs

• The README first-screen story now leads with CI Proof Packets for AI code
  review and links to the v0.1.6 release.

Verification

• Opened a real dogfood PR: #94.
• First ProofFlow PR Review run succeeded and produced:
  • status=completed
  • risk_level=info
  • changed_files=1
  • claims_created=1
  • evidence_created=1
  • downloadable summary.json
  • downloadable Proof Packet markdown
• After the comment polish, the second ProofFlow PR Review run succeeded and
  updated the same comment:
  • changed_files=2
  • claims_created=1
  • evidence_created=1
  • Proof Packet row: Included in workflow artifact proofflow-agentguard-review
• All PR checks passed:
  • AgentGuard Proof Packet
  • Python 3.11
  • Python 3.12
  • Frontend test/build
  • Ruff
  • TypeScript
• Local checks:
  • cd backend && python -m pytest tests/test_release_docs.py
  • git diff --check