feat(config): add review.test_generation toggle (default off) for boundary-safe test generation

[JSONbored]

Config slice for #1972: a per-repo default-OFF flag gating whether a test-gen action spec is emitted when the test-evidence signal reports missing tests, wired through FocusManifestReviewConfig with a global env kill-switch, byte-identical when unset.

Deliverables

• Add testGeneration: boolean|null to FocusManifestReviewConfig + parse/normalize, default null
• isTestGenerationEnabled(env) kill-switch helper (mirror isInlineCommentsEnabled src/review/inline-comments.ts:21)
• Resolver AND-ing manifest toggle + env flag
• Tests: manifest/env combinations, absent defaults, both branches

References

• src/signals/focus-manifest.ts:131
• src/review/inline-comments.ts:21
• src/signals/test-evidence.ts:1

Part of #1972.

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

[linear-code]

JSO-2290