Add Google Antigravity CLI (agy) as a harness via print mode (no native ACP)

[sanirudh17]

Summary

Google Antigravity CLI (agy) currently has no native ACP — the upstream request for --acp (google-antigravity/antigravity-cli#31) remains open — so it cannot plug into Zeron's shared AcpHarness today. The stable automation surface is its print mode (agy -p / --print): spawn agy as a subprocess, feed it one prompt, capture the single output block, and render it as the turn.

Solution: add a print-mode subprocess harness in zeron-harness (the crate that already hosts the claude-code and codex subprocess adapters), behind a stable HarnessId::Antigravity. When upstream #31 lands, the adapter migrates to AcpHarness under the same HarnessId — keeping it behind the Harness trait makes that migration additive.

Print-mode contract

From google-antigravity/antigravity-cli#31, agy v1.0.0 exposes only:

Mode	Streaming	Tool approval	Cancel	Conversation state
Default TUI	✅	✅	✅	✅
-i / --prompt-interactive	✅	✅	✅	✅
-p / --print	❌ (single block)	❌ (--dangerously-skip-permissions only)	❌	New each invocation

-p is the only non-interactive mode, so it is the integration surface.

Solution / proposed scope

• Add HarnessId::Antigravity and a print-mode harness in zeron-harness — spawn agy -p, capture the single output block, and emit it as the turn's assistant message (mirror the existing subprocess adapters; do not route through AcpHarness).
• Binary discovery: AGY_EXECUTABLE / ANTIGRAVITY_EXECUTABLE override + PATH/login-shell discovery. Install: curl -fsSL https://antigravity.google/cli/install.sh | bash (Linux/macOS); install.ps1 / install.cmd (Windows).
• Prompt contract: one turn per invocation, cwd = the session's space; confirm the exact prompt-passing shape from agy --help during implementation.
• Permissions: print mode cannot do interactive tool approval → select --sandbox vs --dangerously-skip-permissions behind Zeron's permission settings; tool calls surface post-hoc in the transcript.
• Auth: agy authenticates via the system keyring with browser fallback, and on SSH/headless prints an authorization URL — slot into the existing agent-accounts credential handling (macOS Keychain via security-framework already exists in the engine).
• Models/options: agy reads ~/.gemini/antigravity-cli/settings.json; expose a model picker by parsing/overriding that config. No ACP configOptions discovery available.
• Image attachments: supported. Images attached in the chat (the composer already supports drag-drop/paste) are forwarded to agy by referencing their local file paths in the -p prompt, and rendered in the transcript via the existing attachment pipeline. Native image paste into the CLI itself is an open upstream gap (Feature Request: Support pasting images directly into the CLI google-antigravity/antigravity-cli#38), so path-based references are the mechanism.
• Steering: turn-boundary only — steers dispatch as the next turn via the existing command plane.
• Cancellation: print mode has no cancel → kill the child process on interrupt.
• Liveness: print mode emits a single block only on completion → long silent turns (compaction / long reasoning) will trip the engine's quiet-settle watchdog; extend the existing quiet-settle exemption (Exempt Pi from the 30s ACP quiet-settle #98) to Antigravity so a live turn isn't force-settled as Failed.
• Register lazily and expose through Settings / harness pickers, matching the other providers.

Known limitations (accepted for this integration)

• No resume / no conversation state in print mode: each invocation starts a fresh session ("New each invocation" per Harness settings page, model prefetch, picker highlight fix #31). Each turn is treated as independent; context is re-established by folding the transcript into the prompt.
• No incremental streaming: the transcript updates at turn completion; the turn stays Working until the child exits — hence the quiet-settle exemption above.
• No native image paste into the CLI: upstream Bootstrap checkpoint heals dep-hole chat2 rooms #38 is open; attached images are forwarded by file path instead and rendered in the transcript.
• Windows: agy ships Windows installers, but Zeron's gpui_platform features in this repo target Linux/macOS — the harness itself is platform-neutral, so coverage/CI can stay Linux-only.

Test coverage

• install/discovery: binary detection, --version, env override
• spawn: cwd + prompt-arg shape
• single-turn run: output block captured as the turn's assistant message
• cancellation: child kill on interrupt
• permission-flag selection behind settings (--sandbox vs --dangerously-skip-permissions)
• settings.json model/options parsing
• attachments: an image attached in the chat is referenced in the agy -p prompt and displayed in the transcript
• quiet-settle exemption: a long silent turn survives the watchdog
• real agy -p smoke test where a signed-in agy is available