feat: Add Antigravity CLI as a supported coding agent

[beingminimal]

Issue Description

Is your feature request related to a problem? Please describe.
Buzz currently supports integrations with select coding agents, but lacks native support for Antigravity CLI (agy / google-antigravity). Developers and automated workflows using Google's Antigravity CLI as their primary terminal-based agent harness cannot currently spawn or manage agy agents within Buzz workspace channels.

Describe the solution you'd like
Add Antigravity CLI as a first-class supported agent runner/harness within Buzz:

1. Agent Detection & Spawn Engine: Support detecting local/remote agy installations and spawning agent instances bound to Buzz channels via BUZZ_PRIVATE_KEY identity scoping.
2. Event & Patch Mapping: Map Antigravity CLI agent actions (multi-file edits, terminal execution, NIP-34 patch proposals) directly into Buzz's Nostr event log.
3. Session & Auth Handling: Integrate session export/import and credential management so Antigravity CLI agents can collaborate seamlessly alongside human teammates in project channels.

Describe alternatives you've considered
Running Antigravity CLI externally in a separate terminal session and manually copying patches and outputs into Buzz channels, which loses the unified audit trail, real-time review integration, and event history.

Additional context

• Antigravity CLI documentation
• Should align with Buzz's identity model where agents operate via their own keypairs and channel memberships.