v0.2.35: fix: flag degenerate review clustering

Warn explicitly when three or more review findings each collapse into singleton clusters, so classifier failure cannot masquerade as an ordinary zero-sweep result.

Fixed

• Emit a receipt advisory that identifies likely prose-hash fallback signatures and requires a manual sweep before fixing.
• Keep the advisory informational; loop control and the re-review cap are unchanged.
• Document the advisory beside the existing Convergence: contract.

Validation

• /opt/homebrew/bin/pytest -q — 796 passed
• ruff check plugins/ tests/ — passed
• Two-axis local review against issue #73 and repository standards — clean after strengthening the multi-site negative test

Closes #73

Summary by Sourcery

Add an explicit advisory to receipts when review clustering degenerates into singleton patterns, and document the new advisory alongside existing convergence guidance.

Bug Fixes:

• Flag degenerate review clustering by emitting a warning when three or more findings all become singleton patterns, distinguishing classifier failures from normal zero-sweep results.

Enhancements:

• Extend receipt output to include a clustering advisory line and clarify its advisory-only role in the skill documentation.

Documentation:

• Document the new Clustering: advisory behavior and its relationship to the existing Convergence: line in the review loop skill guide.

Tests:

• Add tests covering when the degenerate clustering advisory should be emitted or suppressed based on cluster composition.