feat(doctor): add safeguards for town root branch protection

Add three layers of protection to prevent accidental branch switches in
the town root (~/gt), which should always stay on main:

1. Doctor check `town-root-branch`: Verifies town root is on main/master.
   Fixable via `gt doctor --fix` to switch back to main.

2. Doctor check `pre-checkout-hook`: Verifies git pre-checkout hook is
   installed. The hook blocks checkout from main to any other branch.
   Fixable via `gt doctor --fix` or `gt git-init`.

3. Runtime warning in all gt commands: Non-blocking warning if town root
   is on wrong branch, with fix instructions.

The root cause of this issue was git commands running in the wrong
directory, switching the town root to a polecat branch. This broke gt
commands because rigs.json and other configs were on main, not the
polecat branch.

Closes: hq-1kwuj

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

internal/cmd/gitinit.go

@@ -267,6 +267,11 @@ func InitGitForHarness(hqRoot string, github string, private bool) error {
 		fmt.Printf("   ✓ Git repository already exists\n")
 	}
 
+	// Install pre-checkout hook to prevent accidental branch switches
+	if err := InstallPreCheckoutHook(hqRoot); err != nil {
+		fmt.Printf("   %s Could not install pre-checkout hook: %v\n", style.Dim.Render("⚠"), err)
+	}
+
 	// Create GitHub repo if requested
 	if github != "" {
 		if err := createGitHubRepo(hqRoot, github, private); err != nil {
@@ -276,3 +281,102 @@ func InitGitForHarness(hqRoot string, github string, private bool) error {
 
 	return nil
 }
+
+// PreCheckoutHookScript is the git pre-checkout hook that prevents accidental
+// branch switches in the town root. The town root should always stay on main.
+const PreCheckoutHookScript = `#!/bin/bash
+# Gas Town pre-checkout hook
+# Prevents accidental branch switches in the town root (HQ).
+# The town root must stay on main to avoid breaking gt commands.
+
+# Only check branch checkouts (not file checkouts)
+# $3 is 1 for file checkout, 0 for branch checkout
+if [ "$3" = "1" ]; then
+    exit 0
+fi
+
+# Get the target branch name
+TARGET_BRANCH=$(git rev-parse --abbrev-ref "$2" 2>/dev/null)
+
+# Allow checkout to main or master
+if [ "$TARGET_BRANCH" = "main" ] || [ "$TARGET_BRANCH" = "master" ]; then
+    exit 0
+fi
+
+# Get current branch
+CURRENT_BRANCH=$(git branch --show-current)
+
+# If already not on main, allow (might be fixing the situation)
+if [ "$CURRENT_BRANCH" != "main" ] && [ "$CURRENT_BRANCH" != "master" ]; then
+    exit 0
+fi
+
+# Block the checkout with a warning
+echo ""
+echo "⚠️  BLOCKED: Town root must stay on main branch"
+echo ""
+echo "   You're trying to switch from '$CURRENT_BRANCH' to '$TARGET_BRANCH'"
+echo "   in the Gas Town HQ directory."
+echo ""
+echo "   The town root (~/gt) should always be on main. Switching branches"
+echo "   can break gt commands (missing rigs.json, wrong configs, etc.)."
+echo ""
+echo "   If you really need to switch branches, you can:"
+echo "   1. Temporarily rename .git/hooks/pre-checkout"
+echo "   2. Do your work"
+echo "   3. Switch back to main"
+echo "   4. Restore the hook"
+echo ""
+exit 1
+`
+
+// InstallPreCheckoutHook installs the pre-checkout hook in the town root.
+// This prevents accidental branch switches that can break gt commands.
+func InstallPreCheckoutHook(hqRoot string) error {
+	hooksDir := filepath.Join(hqRoot, ".git", "hooks")
+
+	// Ensure hooks directory exists
+	if err := os.MkdirAll(hooksDir, 0755); err != nil {
+		return fmt.Errorf("creating hooks directory: %w", err)
+	}
+
+	hookPath := filepath.Join(hooksDir, "pre-checkout")
+
+	// Check if hook already exists
+	if _, err := os.Stat(hookPath); err == nil {
+		// Read existing hook to see if it's ours
+		content, err := os.ReadFile(hookPath)
+		if err != nil {
+			return fmt.Errorf("reading existing hook: %w", err)
+		}
+
+		if strings.Contains(string(content), "Gas Town pre-checkout hook") {
+			fmt.Printf("   ✓ Pre-checkout hook already installed\n")
+			return nil
+		}
+
+		// There's an existing hook that's not ours - don't overwrite
+		fmt.Printf("   %s Pre-checkout hook exists but is not Gas Town's (skipping)\n", style.Dim.Render("⚠"))
+		return nil
+	}
+
+	// Install the hook
+	if err := os.WriteFile(hookPath, []byte(PreCheckoutHookScript), 0755); err != nil {
+		return fmt.Errorf("writing hook: %w", err)
+	}
+
+	fmt.Printf("   ✓ Installed pre-checkout hook (prevents accidental branch switches)\n")
+	return nil
+}
+
+// IsPreCheckoutHookInstalled checks if the Gas Town pre-checkout hook is installed.
+func IsPreCheckoutHookInstalled(hqRoot string) bool {
+	hookPath := filepath.Join(hqRoot, ".git", "hooks", "pre-checkout")
+
+	content, err := os.ReadFile(hookPath)
+	if err != nil {
+		return false
+	}
+
+	return strings.Contains(string(content), "Gas Town pre-checkout hook")
+}