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/cmd/disable.go

@@ -0,0 +1,72 @@
+// ABOUTME: Command to disable Gas Town system-wide.
+// ABOUTME: Sets the global state to disabled so tools work vanilla.
+
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/shell"
+	"github.com/steveyegge/gastown/internal/state"
+	"github.com/steveyegge/gastown/internal/style"
+)
+
+var disableClean bool
+
+var disableCmd = &cobra.Command{
+	Use:     "disable",
+	GroupID: GroupConfig,
+	Short:   "Disable Gas Town system-wide",
+	Long: `Disable Gas Town for all agentic coding tools.
+
+When disabled:
+  - Shell hooks become no-ops
+  - Claude Code SessionStart hooks skip 'gt prime'
+  - Tools work 100% vanilla (no Gas Town behavior)
+
+The workspace (~/gt) is preserved. Use 'gt enable' to re-enable.
+
+Flags:
+  --clean  Also remove shell integration from ~/.zshrc/~/.bashrc
+
+Environment overrides still work:
+  GASTOWN_ENABLED=1   - Enable for current session only`,
+	RunE: runDisable,
+}
+
+func init() {
+	disableCmd.Flags().BoolVar(&disableClean, "clean", false,
+		"Remove shell integration from RC files")
+	rootCmd.AddCommand(disableCmd)
+}
+
+func runDisable(cmd *cobra.Command, args []string) error {
+	if err := state.Disable(); err != nil {
+		return fmt.Errorf("disabling Gas Town: %w", err)
+	}
+
+	if disableClean {
+		if err := removeShellIntegration(); err != nil {
+			fmt.Printf("%s Could not clean shell integration: %v\n",
+				style.Warning.Render("!"), err)
+		} else {
+			fmt.Println("  Removed shell integration from RC files")
+		}
+	}
+
+	fmt.Printf("%s Gas Town disabled\n", style.Success.Render("✓"))
+	fmt.Println()
+	fmt.Println("All agentic coding tools now work vanilla.")
+	if !disableClean {
+		fmt.Printf("Use %s to also remove shell hooks\n",
+			style.Dim.Render("gt disable --clean"))
+	}
+	fmt.Printf("Use %s to re-enable\n", style.Dim.Render("gt enable"))
+
+	return nil
+}
+
+func removeShellIntegration() error {
+	return shell.Remove()
+}