feat(hooks): add native Gemini CLI target

[RaviTharuma]

Summary

• add geminicli as a native hooks target for current Google Gemini CLI
• safely patch the hooks key in global/project .gemini/settings.json while preserving unrelated settings
• support import/generate round-trips, canonical event mappings, Gemini-specific overrides, command metadata, and docs
• add processor, path, target, schema, and adapter coverage

Compatibility

This is intentionally separate from Antigravity targets. It manages only the native Gemini CLI hooks surface.

Test plan

• pnpm check
• focused Gemini/processor/tool-target tests
• pnpm test — 324 files, 7,272 tests passed

Closes #2318.

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[RaviTharuma]

CI update: the external-contributor policy allows at most two concurrent open PRs. PR #2322 was temporarily closed, leaving only these two open. The branch has also been updated with the CI fixes discovered in the first run.

[dyoshikawa]

Closing along with #2318: the geminicli target was intentionally removed and consolidated into the antigravity targets in #1673 (context: #1663, #1679) after Google's Gemini CLI → Antigravity CLI transition, so re-adding it contradicts that recorded decision. See #2318 for the full reasoning. This PR also currently fails the Code Quality & Tests check and all three E2E jobs.