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/enable.go

@@ -0,0 +1,54 @@
+// ABOUTME: Command to enable Gas Town system-wide.
+// ABOUTME: Sets the global state to enabled for all agentic coding tools.
+
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/state"
+	"github.com/steveyegge/gastown/internal/style"
+)
+
+var enableCmd = &cobra.Command{
+	Use:     "enable",
+	GroupID: GroupConfig,
+	Short:   "Enable Gas Town system-wide",
+	Long: `Enable Gas Town for all agentic coding tools.
+
+When enabled:
+  - Shell hooks set GT_TOWN_ROOT and GT_RIG environment variables
+  - Claude Code SessionStart hooks run 'gt prime' for context
+  - Git repos are auto-registered as rigs (configurable)
+
+Use 'gt disable' to turn off. Use 'gt status --global' to check state.
+
+Environment overrides:
+  GASTOWN_DISABLED=1  - Disable for current session only
+  GASTOWN_ENABLED=1   - Enable for current session only`,
+	RunE: runEnable,
+}
+
+func init() {
+	rootCmd.AddCommand(enableCmd)
+}
+
+func runEnable(cmd *cobra.Command, args []string) error {
+	if err := state.Enable(Version); err != nil {
+		return fmt.Errorf("enabling Gas Town: %w", err)
+	}
+
+	fmt.Printf("%s Gas Town enabled\n", style.Success.Render("✓"))
+	fmt.Println()
+	fmt.Println("Gas Town will now:")
+	fmt.Println("  • Inject context into Claude Code sessions")
+	fmt.Println("  • Set GT_TOWN_ROOT and GT_RIG environment variables")
+	fmt.Println("  • Auto-register git repos as rigs (if configured)")
+	fmt.Println()
+	fmt.Printf("Use %s to disable, %s to check status\n",
+		style.Dim.Render("gt disable"),
+		style.Dim.Render("gt status --global"))
+
+	return nil
+}