feat(config): add review.min_finding_severity threshold knob

[JSONbored]

Add review.min_finding_severity (enum: nitpick | minor | major | critical) that suppresses AI findings below the threshold from the review comment (e.g. hide nitpicks). Deterministic post-filter; null (default) ⇒ every finding shown (byte-identical). No effect on gate blockers — display-only. Decomposed from #1959, uses the existing severity taxonomy.

Deliverables

• Add minFindingSeverity enum field to FocusManifestReviewConfig in src/signals/focus-manifest.ts via normalizeOptionalEnum over the existing severity ladder (critical|major|minor|nitpick)
• Wire parse + reviewConfigToJson + present + EMPTY literals
• Apply the threshold as a display-only filter over findings in src/review/unified-comment.ts, never changing blocker/gate logic
• Unit tests: each threshold level filters correctly, unset shows all, invalid value warns+ignores; branch coverage on the keep/drop arms and the 'blockers unaffected' invariant
• Document review.min_finding_severity in .gittensory.yml.example, noting it is display-only

References

• src/review/unified-comment.ts:59 (severity field critical|major|minor|nitpick)
• src/signals/focus-manifest.ts:422 (normalizeOptionalEnum)
• .gittensory.yml.example
• part of #1959 (epic #1953)

Part of #1959.

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size: S · gittensor:feature

[linear-code]

JSO-2165