feat(review): deterministic fallback categorization for findings (keyword/path heuristics)

[JSONbored]

When the model omits a finding category, infer one deterministically from the finding body keywords and file path (e.g. test path → tests, security keywords → security), instead of a flat default. Pure heuristic function + tests; consumed by the parser default slice.

Deliverables

• Pure inferFindingCategory(body: string, path: string): FindingCategory using keyword sets + path-matchers (src/signals/path-matchers.ts: isTestPath, isDocsFile, isConfigFile)
• Documented precedence (path signal first, then keyword buckets, then a final default)
• Wire as the fallback in the parser only when the model omits/garbles category
• Tests: each keyword bucket hits, test/docs/config path routing, no-signal → default (every branch of the precedence chain)

References

• src/services/ai-review.ts:414
• src/signals/path-matchers.ts:182
• part of #1958

Part of #1958.

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

[linear-code]

JSO-2250