feat(install): add support for Antigravity CLI plugin configuration

[ykalani]

Adds automatic configuration support for the Antigravity CLI (agy) during installation.

If the installer detects an Antigravity setup (looking for ~/.gemini/config or ~/.gemini/antigravity-cli), it will now:

• Wire up the hook & policy: Automatically creates the policy to allow exit_plan_mode without prompting, and registers plannotator under BeforeTool so browser reviews trigger automatically.
• Copy slash commands: Copies the /plannotator-review and /plannotator-annotate commands into Antigravity's plugin directory so they work out of the box.
• Test coverage: Added new unit tests in install.test.ts to verify the paths and config blocks across bash, powershell, and cmd.

[backnotprop]

Thanks!