feat(review): fix-handoff block renderer for review findings (boundary-safe, content-only)

[JSONbored]

Core renderer for #1962: turn a single review finding into a structured, machine-readable 'apply this fix' block (path/line/description/suggested change) that a contributor's OWN local agent can consume. Content only — no server-side write, no execution. Default-OFF via config; public-safe by construction.

Deliverables

• New src/review/fix-handoff.ts exporting a pure builder finding -> {path, line, severity, instruction, suggestedChange?} handoff block (accepts InlineFinding from src/services/ai-review.ts:215)
• Machine-readable rendering (fenced block / HTML comment marker) mirroring the inline-comment body composition in src/review/inline-comments.ts:69
• Explicit boundary note reusing LOCAL_WRITE_BOUNDARY from src/mcp/local-write-tools.ts:8
• Fail-safe: a finding with no commentable line still yields a valid block (path-only)
• Tests: with/without suggestedChange, with/without line, boundary text present, public-safe escaping (branch-counted)

References

• src/mcp/local-write-tools.ts:8
• src/services/ai-review.ts:215
• src/review/inline-comments.ts:69

Part of #1962.

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

[linear-code]

JSO-2276