feat(enrichment): blame-to-PR regression linker analyzer

[JSONbored]

New REES github-light analyzer (the #1499 epic 'Blame-to-PR regression linker' idea) that, for lines this PR modifies/deletes, resolves which prior PR most recently introduced them (via commit history for the touched ranges) and surfaces those originating PRs — giving the reviewer instant context on what the change is altering. Bounded lookups, fail-safe.

Deliverables

• Add BlameLinkFinding ({ file, line, introducedByPr?: number, introducedByShaPrefix?: string }) + blameLink? key to review-enrichment/src/types.ts (sha prefix only)
• Create review-enrichment/src/analyzers/blame-link.ts: for a bounded set of changed files/ranges, walk commit history for the path and map the most recent introducing commit to its PR via the commits/PR-association API (injected fetch); strict maxFilesProbed + maxLookups caps; emit partial on cap
• Register a descriptor (category 'history', cost 'github-light', requires ['files','github-token']) in review-enrichment/src/analyzers/registry.ts with inline render()
• Add review-enrichment/test/blame-link.test.ts covering a resolved originating PR, an unresolvable line (no finding), the lookup cap, and the token-absent skip

References

• review-enrichment/src/analyzers/commit-signature.ts (github-light commits-API analyzer template)
• review-enrichment/src/analyzers/churn-hotspot.ts (per-file commit-history probe with caps)
• gh issue 1499 lists 'Blame-to-PR regression linker' as a later idea

Part of #1499.

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size: S · gittensor:feature

[linear-code]

JSO-2151