feat(git): warn about co-change partners missing from diff and PR

[devin-ai-integration]

Git history now identifies missing companion edits in diff and PR context—for example, a schema change without its usual migration. The existing co_change_partners contract is preserved: file is the changed anchor, partner is its missing companion, and together is their shared commit count.

Fixes #3015

The canonical Git co-change miner now applies coupling thresholds (3 shared commits / 50% for multiple changed files; 5 / 75% for one), skips commits touching over 50 files, and checks that companions still exist in the selected tree. Mining remains bounded to 1,000 commits within 18 months and eight returned partners. It supports project subdirectories and native Git path bytes, cancellation, and request deadlines. PR context mines history through the merge base and checks file existence at the PR head.

Diff context and all PR-context result variants use the same miner. Unavailable history produces typed co_change_unavailable metadata while preserving available graph context. The duplicate branch-local miner and incompatible response fields were removed; the TypeScript SDK and embedded extension were regenerated from canonical contracts.

Validation on the merged source:

• Runtime Git fixtures: 6 passed, covering coupling thresholds, deletion, bulk commits, subdirectories, unusual path bytes, cancellation, and unavailable history.
• Production MCP journeys: 2 passed, covering exact companion results in diff/PR context, missing-companion removal, unavailable-history enrichment, changed symbols, and invalid input.
• SDK tests: 37 passed; SDK typecheck/build, canonical contracts generation/check, embedded extension build/check, formatting, and diff checks passed.
• Scoped Clippy reached an existing match_bool warning in handlers/analysis/hotspots.rs:73, unchanged from the merged master revision. Its correction is owned by the master refresh. Full workspace tests were not run locally.

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[changeset-bot]

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Latest commit: fde6ab0

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