v0.2.37: feat: add duplicate-block mirror sweep

Summary

• preserve flagged ranges in cluster sites and accept path:start-end sweep inputs
• normalize and fingerprint a single flagged multi-line block, then search only changed files for exact copies
• report exact hits as mirror candidates while retaining the existing token candidate path
• exclude flagged and overlapping ranges from both mirror and token results
• cover range clustering and mirror detection with small repository-owned synthetic corpora

Validation

• /opt/homebrew/bin/pytest -q — 838 passed
• /opt/homebrew/bin/ruff check plugins/gh-review-loop/skills/gh-review-loop/scripts tests
• git diff --check
• repository-wide relationship scan — clean
• recorded replay output remains byte-stable

Depends on the merged #79 range metadata support.

Closes #77.

Summary by Sourcery

Add exact duplicate-block mirror sweeps for multi-line findings and distinguish them from existing token-based sibling detection.

New Features:

• Support parsing and preserving flagged multi-line ranges in sweep inputs and cluster sites.
• Introduce mirror candidate detection by normalizing, fingerprinting, and matching multi-line code blocks across changed files.
• Label sweep candidates by class (mirror vs token) and surface this in reports and CLI usage.

Bug Fixes:

• Exclude flagged and overlapping ranges from both mirror and token sweep results.
• Prevent sweeps from reading files or sites outside the declared changed-file set.

Enhancements:

• Refine sweep status handling so a successful mirror match can be reported even when token-based patterns are too thin or have too few sites.
• Share candidate truncation and sorting logic across mirror and token sweeps.
• Improve report output to omit empty invariant-token sections and to explain truncation or fallback reasons more clearly.

Documentation:

• Document ranged-site usage, mirror sweeps, and their interaction with token-based sweeps in the review-loop skill guide.

Tests:

• Use repository-owned synthetic fixtures for mirror detection, span clustering, punctuation significance, range parsing, comment stripping, overlap exclusion, and report labelling.