Adds Google's Antigravity CLI (`agy`) as the 12th supported coding-tool
runtime, alongside Claude / Codex / Cursor / Copilot / Gemini / Hermes /
Kimi / Kiro / OpenCode / OpenClaw / Pi.
The CLI emits plain assistant text on stdout (no structured event
stream), so the backend streams stdout line-by-line as `MessageText`
events and accumulates the same text as the final `Result.Output`.
Session resumption uses `--conversation <id>`; because the conversation
UUID is not echoed on stdout, the daemon routes `--log-file` to a temp
file and recovers the id from the glog-formatted log lines.
MUL-2767
Co-authored-by: multica-agent <github@multica.ai>
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