v0.2.36: fix: make shape clustering span-aware

Summary

• fetch review range starts at thread and comment level, with single-line fallback
• cluster on the union of code tokens across each anchored span
• require shared identifier-like tokens at both shape and sweep safety floors
• add a captured 16-finding SignalScout #195 corpus with matching historical source snapshots

Validation

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  822 passed in 6.38s — 822 passed
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  10 passed in 0.36s — 10 passed; recorded replay output remains byte-stable
• All checks passed!

Closes #74.
Closes #75.
Closes #76.

Summary by Sourcery

Make review-shape clustering span-aware and tighten token-based safety floors for clustering and sibling sweeps.

New Features:

• Support span-anchored clustering by reading all code lines in a review range instead of a single line.
• Include GitHub review range start fields in thread fetching and propagate them into captured findings.
• Add a captured SignalScout PR195 corpus with anchors and matching historical source snapshots for real-world clustering coverage.

Enhancements:

• Cluster shapes and sweep siblings only when shared tokens include identifier-like signal rather than punctuation-only intersections.

Documentation:

• Update the GH review loop skill documentation to explain span-based shape clustering, single-line fallback, and the requirement for letter-bearing shared tokens in sweep patterns.

Tests:

• Extend clustering, sweep sibling, and GitHub thread fetching tests with PR195 corpus cases and new assertions around span unioning, significance-filtered tokens, and GraphQL query fields.