F-antigravity-sessions: Process Spawning & UI Integration

[bastien-gallay]

Issue #146 — Antigravity Process Spawning & UI Integration

Integrate Antigravity into TermHerd's process lifecycle and UI widgets.

Background

Users should be able to launch fresh Antigravity (agy) sessions and resume
existing ones from the sidebar, just as they do with Claude Code.

Requirements

1. Update Launch enum in crates/core/src/app.rs:

   pub enum Launch {
       Shell,
       Claude { resume: Option<String> },
       Antigravity { resume: Option<String> },
   }

2. In crates/pty/src/lib.rs, map Launch::Antigravity to typing the launch
   command in the terminal shell:

   • Launch::Antigravity { resume: None } => "agy\r"
   • Launch::Antigravity { resume: Some(id) } => "agy --conversation <id>\r"

3. Update crates/app/src/shell.rs to render a new "Launch Antigravity"
   button/affordance next to the shell $ and Claude 🤖 buttons in the
   repository list in the sidebar.

4. Wire up the tab & sidebar list: double-clicking or clicking an Antigravity
   session resumes it.

5. Display the correct display title on the tabs and sidebar using the
   session's parsed title/summary.

Tests

• Spawning a fresh/resumed Antigravity session correctly types agy/agy --conversation <id> into the terminal.
• Launch UI button triggers the correct event and starts the PTY.

[bastien-gallay]

Refactor prerequisites: #167 (B3 shell/launch.rs + B4 tab-title policy → core) and #169 (A3 Sessions registry).