feat(antigravity): add native plan review and installer support

[ykalani]

Adds native Antigravity CLI plan-file review and installer support.

When Antigravity is detected, the installer writes ~/.gemini/config/plugins/plannotator/{plugin.json,hooks.json} and installs review/annotation skills. ~/.gemini/antigravity-cli is only a detection signal, never an installation destination.

The named PreToolUse hook intercepts writes to the current conversation's implementation_plan.md. The runtime reads Antigravity's camelCase toolCall payload, opens Plannotator with the antigravity origin, and returns native allow/deny decisions with feedback. Incremental plan edits request a full-file resubmission; unrelated writes abstain with {}.

Installation safeguards:

• CLI, environment, and config opt-outs; detected/skipped/not-detected reporting; uninstall cleanup, including paths left by the original integration.
• Probe the downloaded binary before writing Antigravity configuration, so old Plannotator releases are skipped. Hooks reference that exact binary by absolute path.
• Antigravity-specific skills set PLANNOTATOR_ORIGIN=antigravity for review and annotation.
• Antigravity-only profiles do not receive Gemini configuration merely because they share ~/.gemini. Profiles with Gemini-specific signals retain both integrations.

Scope and compatibility:

• This gates conventional plan-file writes, not a planning-to-implementation transition. Shell writes, custom plan filenames, and Antigravity's own artifact-review settings remain outside this hook.
• Antigravity CLI 1.2.7+ is the documented support baseline. A live session acceptance check and maintainer agreement on the plan-write gate remain outstanding; this PR does not claim end-to-end verification.
• This is a minor-release feature; no release version is bumped here.

Validation covers temporary-home installs in Bash, PowerShell, and CMD; old-binary preservation; Gemini coexistence; repeat installs; quoted binary paths; opt-outs; uninstall; and native payload/decision handling. Installed plugin shape is also checked with agy plugin validate when available. See apps/antigravity/README.md for the live acceptance checklist.

[backnotprop]

Thanks!

[backnotprop]

Thanks @ykalani, and sorry this stalled after a one-word reply. Checked it against main: it is not superseded. The shipped Gemini leg writes ~/.gemini/policies/plannotator.toml plus a BeforeTool hook into ~/.gemini/settings.json (scripts/install.sh:1913-1974, from #384, v0.16.7), while your branch targets Antigravity's distinct plugin layout — ~/.gemini/{config|antigravity-cli}/plugins/plannotator/{plugin.json,hooks.json} and its own policies/ dir. That is genuinely additive.

The branch is 507 commits behind and scripts/install.sh has been restructured substantially since June, so it needs a rebase. Three conventions landed after you opened it that a new host leg is now expected to follow:

1. An opt-out: --skip-antigravity + PLANNOTATOR_SKIP_ANTIGRAVITY_INSTALL + skipInstall.antigravity, matching the codex/gemini/kiro/opencode pattern (scripts/install.sh:296-311, packages/shared/config.ts:163-168).
2. Honest reporting: the installer now prints detected / skipped (reason) / not detected per host rather than an unconditional banner — the unconditional ANTIGRAVITY CLI USERS block in the current diff would print even when nothing was detected.
3. Uninstall: host integrations are removed by packages/server/uninstall.ts; a new leg should add its paths there (see Close remaining uninstall host-integration recovery gaps #1179 for the remaining gaps).

Are you up for a rebase? If we don't hear back in 14 days I'll close this to keep the queue honest — it can be reopened any time, and the design above is the whole review.

[ykalani]

Rebased onto current main and pushed the update to this PR.

Added all three requested conventions:

• Opt-outs via --skip-antigravity (-SkipAntigravity in PowerShell), PLANNOTATOR_SKIP_ANTIGRAVITY_INSTALL, and skipInstall.antigravity.
• Detection reporting that distinguishes installed, skipped with reason, and not detected.
• Uninstall cleanup for both Antigravity layouts, preserving unrelated host files.

Installer and uninstall tests: 197 passed, 9 platform skips. Coverage includes Bash, PowerShell, and CMD, opt-out precedence, repeat installs, and preservation of existing files when skipped.

Ready for another review. Thanks for the guidance!

[backnotprop]

Thanks for the rebase, the opt-out, reporting, and uninstall work is exactly right and should stay. But checked against the Antigravity CLI itself, the plugin it writes can't fire:

• Antigravity's hooks.json has no "hooks" wrapper and uses PreToolUse, not BeforeTool (the binary has zero BeforeTool strings).
• There is no exit_plan_mode tool in Antigravity, so the matcher matches nothing. Which event should Plannotator gate on is the open question.
• policies/ is Gemini CLI's mechanism; Antigravity has none, so policies/plannotator.toml is a dead file, and plugins/<name>/commands/ isn't part of its plugin layout either.
• The only global config root it scans is ~/.gemini/config/; ~/.gemini/antigravity-cli/ is its private data dir (its own changelog calls writing there a bug).
• There's no runtime side: apps/hook/server/index.ts only understands Claude Code and Gemini payloads, so even a firing hook would exit with "No plan content".

So this needs the runtime adapter (payload branch + an antigravity origin) and the corrected plugin shape before it can do anything. Happy to keep it open if you want to take that on; it's a minor-release feature either way.