feat(review): add category enum to AI finding schema + parser default

[JSONbored]

Tag every inline finding with a category (security/correctness/performance/maintainability/tests/style) parsed from the model output with a safe default. Schema + parser + prompt only — rendering and deterministic fallback are separate slices. Unblocks category-level analytics.

Deliverables

• Add a FindingCategory union + optional category on InlineFinding (src/services/ai-review.ts:215)
• Parse category in toInlineFindings (src/services/ai-review.ts:414): accept only known enum values, default to a safe 'maintainability' (or 'other') otherwise
• Update INLINE_FINDINGS_SUFFIX prompt (src/services/ai-review.ts:497) to request the category field
• Preserve category through composeInlineFindings (src/services/ai-review.ts:841)
• Tests: each valid category, unknown/absent → default (both branches), preserved through compose

References

• src/services/ai-review.ts:215
• src/services/ai-review.ts:414
• src/services/ai-review.ts:497
• part of #1958

Part of #1958.

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

[linear-code]

JSO-2249