feat: add agy-acp adapter for Google Antigravity CLI

[chaodu-agent]

Summary

Add a thin Rust ACP stdio adapter (agy-acp) that bridges openab to Google's Antigravity CLI (agy) via agy -p invocations.

Inspired by #863 (@Joseph19820124) — this implementation takes a different approach: a standalone Rust binary instead of a Node.js wrapper, with proper stdin/stdout pipe handling for reliable subprocess communication.

Changes

• agy-acp/: standalone Rust crate (same pattern as gateway/, xai-proxy/)
  • Reads ACP JSON-RPC on stdin from openab
  • Filters <sender_context> blocks, extracts user text
  • Spawns agy -p "prompt" with stdin=null, stdout=piped
  • Uses --continue for multi-turn session continuity
  • Sends response as agent_message_chunk notification
• Dockerfile.antigravity: multi-stage build (openab + agy-acp + agy binary)
• docs/antigravity.md: configuration and usage guide

Config

[agent]
command = "agy-acp"
args = []
working_dir = "/home/agent"

Test plan

• Tested end-to-end locally: openab + agy-acp + agy (周嘟嘟 bot on Discord)
• ~5s response time per prompt
• Multi-turn via --continue verified

Limitations

• No streaming (agy -p returns full response at once)
• Cancel is a no-op (agy -p runs to completion)
• Session continuity uses --continue (resumes most recent conversation)

Discord Discussion URL: https://discord.com/channels/1491295327620169908/1491365150664560881/1507210905862017227

[chaodu-agent]

Fixes pushed (2 commits)

4534a6c — fix(agy-acp): add --dangerously-skip-permissions, stderr capture, timeout

• --dangerously-skip-permissions now passed to agy (matches docs)
• stderr captured and logged via eprintln! for debugging
• Subprocess timeout: 120s default, configurable via AGY_TIMEOUT_SECS
• README.md updated: Antigravity added to intro, features, ASCII diagram, agents table

a4f96f0 — fix(agy-acp): check exit code before marking session history

• Non-zero exit → JSON-RPC error with exit code + stderr message
• has_history only set on success → prevents broken --continue chains after failures

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All issues from @普渡法師 review addressed. CI running.