feat(config): validate .gittensory.yml against schema (fail clearly pre-review)

[JSONbored]

Add an explicit manifest validation pass that surfaces every parse warning as a single grouped, actionable message before a review runs, so invalid config values fail clearly (an acceptance criterion of #1670: 'invalid config values fail clearly before a review runs'). Deterministic; a fully-valid manifest ⇒ byte-identical (no message).

Deliverables

• Add a helper that collects the existing parseFocusManifest warnings[] and formats them into one grouped, deduped advisory surface (reuse the warnings already produced by every parser)
• Emit the grouped validation notice on the review path when warnings exist; suppress entirely when there are none (byte-identical)
• Unit tests: multiple warnings grouped, no warnings = no message, ordering stable; branch coverage on the has-warnings/no-warnings arms
• Document manifest validation behavior in the config docs

References

• src/signals/focus-manifest.ts:305 (warnings accumulation)
• src/signals/focus-manifest.ts:1013 (parseFocusManifest)
• part of #1670 (epic #1680)

Part of #1670.

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size: M · gittensor:feature

[linear-code]

JSO-2173