v0.4.0: Antigravity adapter

What's New

• Antigravity adapter — 9th agent adapter added with full session scanning and parsing from ~/.gemini/antigravity/brain/
• Resume disabled for Antigravity — Antigravity sessions cannot be resumed from Orbit (UI disabled accordingly)

Technical

• Added AntigravityAdapter implementing the AgentAdapter trait with JSONL transcript parsing, tool call extraction, and token estimation
• Added Antigravity variant to AgentType enum with violet color scheme
• Updated ActionBar to handle Antigravity resume restrictions
• Version bumped to 0.4.0