feat: add Antigravity CLI agent support

[agentool]

Summary

• add a bundled ACP compatibility bridge for Antigravity CLI print mode
• register AGY as a managed runtime across discovery, launch configuration, onboarding, and test mocks
• install workspace hooks for ACP observer tool events while preserving existing user hooks
• resume multi-turn work with AGY conversation IDs, with fail-closed trajectory discovery as a compatibility fallback
• discover selectable models through the native agy models command and pass selections through BUZZ_AGY_MODEL

Implementation notes

• Buzz launches AGY with the ACP workspace registered through --add-dir, which enables the workspace's .agents/hooks.json, skills, and rules.
• PreToolUse, PostToolUse, and Stop events are translated into ACP observer updates.
• Completed conversations resume with AGY's documented --conversation option.
• The bridge remains isolated behind buzz-acp agy-acp until Antigravity provides an official ACP/A2A transport.

Validation

• real AGY 1.1.6 two-turn smoke test, including hook-only conversation capture and resume with the trajectory fallback disabled
• real signed-in agy models discovery, returning 11 selectable models through the Desktop response
• cargo test -p buzz-acp
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• focused Desktop runtime, hook-merge, model-normalization, and onboarding tests
• repository pre-commit and pre-push gates, including 1,631 Desktop backend tests

Closes #2393