Add Antigravity CLI support

[aserper]

Fixes #168

Summary

Adds first-class Antigravity CLI (agy) support:

• adds an agy agent profile for prompt injection, auto mode, auto-naming, and continue/resume flags
• classifies agy panes in the sidebar/dashboard with an Antigravity label/color and a reliable AG default icon
• preserves useful pane metadata by rendering Antigravity hostname/framework titles as AG · <title> instead of suppressing them
• resolves the Antigravity config directory as ~/.gemini/antigravity-cli
• installs Antigravity status tracking via the official statusLine signal, with lifecycle hooks as a compatibility/working fallback
• keeps set-window-status silent on success so Antigravity's fail-closed hook parser does not reject the hook output
• refreshes persisted pane title/identity from live tmux data to avoid stale sidebar metadata after pane reuse
• documents Antigravity support and status-tracking setup

Status tracking behavior

workmux setup --hooks now installs:

• a silent statusLine wrapper at ~/.gemini/config/workmux-hooks/workmux-statusline
• statusLine.command / statusLine.enabled = true in ~/.gemini/antigravity-cli/settings.json
• no-argument hook wrapper scripts for fallback lifecycle hooks
• hook groups in both ~/.gemini/config/hooks.json and ~/.gemini/antigravity-cli/hooks.json

The statusLine wrapper reads Antigravity's agent_state JSON payload and maps it to Workmux status. It prints nothing unless it is chaining a user-provided statusLine command, so it should not visibly change the Antigravity UI. If a user already had a custom statusLine command, Workmux stores and chains it so their visible statusLine output is preserved.

Existing running agy sessions may need /statusline on or a restart to reload the new settings.

Plan conformance

This follows the implementation plan saved in the Obsidian note "Workmux Antigravity CLI Implementation Plan":

• implemented AntigravityProfile using canonical profile name agy
• added known-agent detection and prompt-injection tests
• added sidebar/dashboard identity via AgentKind::Antigravity
• added config directory resolution for ~/.gemini/antigravity-cli
• kept sandbox support deferred
• updated docs for agents/configuration/add/status-tracking

The note originally recommended deferring status hooks until Antigravity's hook schema/location was verified. After that plan was written, the lifecycle hooks and the official statusLine payload were verified locally, so this PR includes the planned follow-up status tracking work as well.

Validation

• cargo test — 1268 passed, 4 ignored
• cargo test antigravity — 11 passed
• cargo build --release
• installed release binary to /opt/brew/bin/workmux and /home/amit/.pi/agent/bin/workmux
• local agy tmux smoke test verified spinner starts while working and stops after /statusline on enabled the Workmux statusLine wrapper

[aserper]

Updated after local sidebar testing: the Antigravity default icon is now ⋂, and hostname-only pane titles are filtered so they don't render as the task title for agy panes. Rebuilt successfully with cargo build --release; targeted sidebar identity tests and git diff --check pass.

[aserper]

Updated with the final Antigravity lifecycle fix: status tracking now uses agy's official statusLine agent_state payload as the source of truth, with silent hook wrappers as fallback.

Verified locally with a live agy tmux session after running /statusline on; spinner now clears to done. Full test suite: 1268 passed; 4 ignored.