feat(review): deterministic changed-symbol extraction from the PR diff (impact-map input)

[JSONbored]

Foundational input slice of #1971: a pure extractor that, from the changed files + patches, deterministically pulls the changed/added/removed top-level symbols (exported functions/classes/consts) per file. No RAG query, no rendering — just the symbol set the impact computation consumes. Reuses the RagBoundary vocabulary.

Deliverables

• New src/review/impact-symbols.ts exporting extractChangedSymbols(files: {path, patch}[]) -> {path, symbols: string[]}[]
• Language-aware but bounded (TS/JS export forms first; degrade gracefully for others)
• Aligns with RagBoundary ('function'|'class'|'export') in src/review/rag.ts:105
• Fail-safe: unparseable patch yields empty symbol set, never throws
• Tests: added export, removed export, modified body, no-symbol diff, malformed patch (branch-counted)

References

• src/review/rag.ts:105
• src/review/rag-wire.ts:144
• src/review/inline-comments.ts:49

Part of #1971.

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2283