feat(enrichment): public-API undocumented-export scan analyzer

[JSONbored]

New REES github-light analyzer (the #1499 epic 'Public-API undocumented-export scan' idea) that flags exports newly added to a package's public entrypoint (main/exports/index) that ship with no adjacent doc comment — undocumented public surface. Fetches the changed entrypoint file at headSha to see the doc-block/signature pairing; conservative, fail-safe.

Deliverables

• Add UndocumentedExportFinding ({ file, line, symbol: string }) + undocumentedExport? key to review-enrichment/src/types.ts
• Create review-enrichment/src/analyzers/undocumented-export.ts: identify added exports in public-entrypoint files (from package.json main/exports or index.*), fetch the file at headSha (injected fetch) to check for a preceding JSDoc/docstring, flag those without; maxFiles + maxFindings caps
• Register a descriptor (category 'quality', cost 'github-light', requires ['files','github-token','head-sha']) in review-enrichment/src/analyzers/registry.ts with inline render()
• Add review-enrichment/test/undocumented-export.test.ts covering an undocumented public export flagged, a documented one not flagged, a non-entrypoint file skipped, and the token-absent skip

References

• review-enrichment/src/analyzers/doc-comment-drift.ts (github-light fetch-file-at-headSha + doc/signature parse template)
• review-enrichment/src/analyzers/registry.ts:373-392
• gh issue 1499 lists 'Public-API undocumented-export scan' as a later idea

Part of #1499.

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size: S · gittensor:feature

[linear-code]

JSO-2152