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/shell.go

@@ -0,0 +1,99 @@
+// ABOUTME: Shell integration management commands.
+// ABOUTME: Install/remove shell hooks without full HQ setup.
+
+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 shellCmd = &cobra.Command{
+	Use:     "shell",
+	GroupID: GroupConfig,
+	Short:   "Manage shell integration",
+	RunE:    requireSubcommand,
+}
+
+var shellInstallCmd = &cobra.Command{
+	Use:   "install",
+	Short: "Install or update shell integration",
+	Long: `Install or update the Gas Town shell integration.
+
+This adds a hook to your shell RC file that:
+  - Sets GT_TOWN_ROOT and GT_RIG when you cd into a Gas Town rig
+  - Offers to add new git repos to Gas Town on first visit
+
+Run this after upgrading gt to get the latest shell hook features.`,
+	RunE: runShellInstall,
+}
+
+var shellRemoveCmd = &cobra.Command{
+	Use:   "remove",
+	Short: "Remove shell integration",
+	RunE:  runShellRemove,
+}
+
+var shellStatusCmd = &cobra.Command{
+	Use:   "status",
+	Short: "Show shell integration status",
+	RunE:  runShellStatus,
+}
+
+func init() {
+	shellCmd.AddCommand(shellInstallCmd)
+	shellCmd.AddCommand(shellRemoveCmd)
+	shellCmd.AddCommand(shellStatusCmd)
+	rootCmd.AddCommand(shellCmd)
+}
+
+func runShellInstall(cmd *cobra.Command, args []string) error {
+	if err := shell.Install(); err != nil {
+		return err
+	}
+
+	if err := state.Enable(Version); err != nil {
+		fmt.Printf("%s Could not enable Gas Town: %v\n", style.Dim.Render("⚠"), err)
+	}
+
+	fmt.Printf("%s Shell integration installed (%s)\n", style.Success.Render("✓"), shell.RCFilePath(shell.DetectShell()))
+	fmt.Println()
+	fmt.Println("Run 'source ~/.zshrc' or open a new terminal to activate.")
+	return nil
+}
+
+func runShellRemove(cmd *cobra.Command, args []string) error {
+	if err := shell.Remove(); err != nil {
+		return err
+	}
+
+	fmt.Printf("%s Shell integration removed\n", style.Success.Render("✓"))
+	return nil
+}
+
+func runShellStatus(cmd *cobra.Command, args []string) error {
+	s, err := state.Load()
+	if err != nil {
+		fmt.Println("Gas Town: not configured")
+		fmt.Println("Shell integration: not installed")
+		return nil
+	}
+
+	if s.Enabled {
+		fmt.Println("Gas Town: enabled")
+	} else {
+		fmt.Println("Gas Town: disabled")
+	}
+
+	if s.ShellIntegration != "" {
+		fmt.Printf("Shell integration: %s (%s)\n", s.ShellIntegration, shell.RCFilePath(s.ShellIntegration))
+	} else {
+		fmt.Println("Shell integration: not installed")
+	}
+
+	return nil
+}