feat: Set and Forget - Seamless Gas Town Integration (#255)

Adds shell integration for automatic Gas Town context detection.

Features:
- `gt enable` / `gt disable` - Global on/off switch
- `gt shell install|remove|status` - Shell integration management
- `gt rig quick-add [path]` - One-command project setup
- `gt uninstall` - Clean removal with options
- Shell hook auto-sets GT_TOWN_ROOT/GT_RIG on cd

Implementation:
- XDG-compliant state storage (~/.local/state/gastown/)
- Safe RC file manipulation with block markers
- Environment overrides (GASTOWN_DISABLED/ENABLED)
- Doctor check for global state validation

Co-authored-by: Sohail Mohammad <sohailm25@gmail.com>

internal/config/types.go

@@ -225,8 +225,9 @@ type RuntimeConfig struct {
 	Command string `json:"command,omitempty"`
 
 	// Args are additional command-line arguments.
-	// Default: ["--dangerously-skip-permissions"]
-	Args []string `json:"args,omitempty"`
+	// Default: ["--dangerously-skip-permissions"] for built-in agents.
+	// Empty array [] means no args (not "use defaults").
+	Args []string `json:"args"`
 
 	// InitialPrompt is an optional first message to send after startup.
 	// For claude, this is passed as the prompt argument.