feat(config): add review.shared_config operator base-manifest overlay

[JSONbored]

Add review.shared_config — an operator-level base manifest (in the container-private config dir) that a per-repo manifest overlays, so operators set review defaults across many repos with minimal per-repo YAML. Absent ⇒ byte-identical single-manifest behavior. Decomposed from #1959; builds on GITTENSORY_REPO_CONFIG_DIR.

Deliverables

• Load an optional shared/base manifest from the container-private config dir (GITTENSORY_REPO_CONFIG_DIR) and overlay the per-repo review: config over it (per-field: repo value wins, else base value, else default)
• Add merge logic + a sharedConfigSource provenance note; keep the existing manifest resolution precedence (.gittensory.yml > dashboard > defaults) intact
• Unit tests: base-only, repo-overrides-base per field, absent base = byte-identical, malformed base warns+ignores; branch coverage on every overlay arm
• Document the overlay precedence + private-config placement in .gittensory.yml.example and the manifest reference docs

References

• src/config/gittensory-repo-focus-manifest.ts
• src/signals/focus-manifest.ts:653 (parseReviewConfig)
• src/signals/focus-manifest.ts:196 (review config on FocusManifest)
• part of #1959 / #1680 (epic #1953)

Part of #1959.

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size: M · gittensor:feature

[linear-code]

JSO-2163