Add system tray icon with menu for agent

- Added tray-icon and muda crates for tray functionality
- Tray icon shows green circle when connected
- Menu displays: session code, machine name, End Session option
- End Session menu item cleanly terminates the agent
- Tray events processed in session main loop

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agent/Cargo.toml

@@ -51,6 +51,13 @@ hostname = "0.4"
 # URL encoding
 urlencoding = "2"
 
+# System tray (Windows)
+tray-icon = "0.19"
+muda = "0.15"  # Menu for tray icon
+
+# Image handling for tray icon
+image = { version = "0.25", default-features = false, features = ["png"] }
+
 [target.'cfg(windows)'.dependencies]
 # Windows APIs for screen capture and input
 windows = { version = "0.58", features = [