feat(config): add review.fixHandoff toggle (default off) for fix-handoff blocks

[JSONbored]

Config slice for #1962: a per-repo boolean under review that gates whether fix-handoff blocks are emitted, wired through FocusManifestReviewConfig with a global env kill-switch, default-OFF, byte-identical when unset. Follows the inline_comments toggle precedent exactly.

Deliverables

• Add fixHandoff: boolean|null to FocusManifestReviewConfig + parse/normalize (mirror inlineComments in src/signals/focus-manifest.ts:131)
• isFixHandoffEnabled(env) env kill-switch helper mirroring isInlineCommentsEnabled in src/review/inline-comments.ts:21
• shouldEmitFixHandoff() AND-ing manifest toggle + env flag (mirror shouldRequestInlineFindings src/review/inline-comments.ts:29)
• Tests: all three-way combinations of manifest/env, absent defaults, both branches

References

• src/signals/focus-manifest.ts:131
• src/review/inline-comments.ts:21
• src/review/inline-comments.ts:29

Part of #1962.

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size: XS · held for later release (gittensor:bug once activated)

[linear-code]

JSO-2277