Add studio tool for multi-agent workspace management

Studio enables running multiple g3 agents concurrently without conflicts
by using git worktrees for isolation.

Features:
- studio run --agent <name> [args...]: Create worktree, spawn g3, tail output
- studio list: Show all active sessions
- studio status <id>: Show session details and summary
- studio accept <id>: Merge session branch to main and cleanup
- studio discard <id>: Delete session without merging

Each session gets:
- Isolated worktree at .worktrees/sessions/<agent>/<session-id>
- Dedicated branch: sessions/<agent>/<session-id>
- Short UUID (8 chars) for easy reference
- Automatic --workspace and --agent flags passed to g3

crates/studio/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "studio"
+version = "0.1.0"
+edition = "2021"
+description = "Multi-agent workspace manager for G3"
+
+[[bin]]
+name = "studio"
+path = "src/main.rs"
+
+[dependencies]
+clap = { workspace = true }
+tokio = { workspace = true }
+anyhow = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+uuid = { workspace = true }
+chrono = { version = "0.4", features = ["serde"] }