fix(handoff): use env var fallback when town root detection fails

When the repo is in a broken state (wrong branch, detached HEAD, deleted
worktree), gt handoff would fail with "cannot detect town root" error.
This is exactly when handoff is most needed - to recover and hand off
to a fresh session.

Changes:
- detectTownRootFromCwd() now falls back to GT_TOWN_ROOT and GT_ROOT
  environment variables when cwd-based detection fails
- buildRestartCommand() now propagates GT_ROOT to ensure subsequent
  handoffs can also use the fallback
- Added tests for the fallback behavior

Fixes gt-x2q81.

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

internal/cmd/handoff.go

@@ -411,6 +411,10 @@ func buildRestartCommand(sessionName string) (string, error) {
 		}
 	}
 
+	// Propagate GT_ROOT so subsequent handoffs can use it as fallback
+	// when cwd-based detection fails (broken state recovery)
+	exports = append(exports, "GT_ROOT="+townRoot)
+
 	// Preserve GT_AGENT across handoff so agent override persists
 	if currentAgent != "" {
 		exports = append(exports, "GT_AGENT="+currentAgent)
@@ -498,14 +502,33 @@ func sessionToGTRole(sessionName string) string {
 }
 
 // detectTownRootFromCwd walks up from the current directory to find the town root.
+// Falls back to GT_TOWN_ROOT or GT_ROOT env vars if cwd detection fails (broken state recovery).
 func detectTownRootFromCwd() string {
 	// Use workspace.FindFromCwd which handles both primary (mayor/town.json)
 	// and secondary (mayor/ directory) markers
 	townRoot, err := workspace.FindFromCwd()
-	if err != nil {
+	if err == nil && townRoot != "" {
-		return ""
-	}
 		return townRoot
+	}
+
+	// Fallback: try environment variables for town root
+	// GT_TOWN_ROOT is set by shell integration, GT_ROOT is set by session manager
+	// This enables handoff to work even when cwd detection fails due to
+	// detached HEAD, wrong branch, deleted worktree, etc.
+	for _, envName := range []string{"GT_TOWN_ROOT", "GT_ROOT"} {
+		if envRoot := os.Getenv(envName); envRoot != "" {
+			// Verify it's actually a workspace
+			if _, statErr := os.Stat(filepath.Join(envRoot, workspace.PrimaryMarker)); statErr == nil {
+				return envRoot
+			}
+			// Try secondary marker too
+			if info, statErr := os.Stat(filepath.Join(envRoot, workspace.SecondaryMarker)); statErr == nil && info.IsDir() {
+				return envRoot
+			}
+		}
+	}
+
+	return ""
 }
 
 // handoffRemoteSession respawns a different session and optionally switches to it.

internal/cmd/handoff_test.go

@@ -0,0 +1,124 @@
+package cmd
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/steveyegge/gastown/internal/workspace"
+)
+
+func TestDetectTownRootFromCwd_EnvFallback(t *testing.T) {
+	// Save original env vars and restore after test
+	origTownRoot := os.Getenv("GT_TOWN_ROOT")
+	origRoot := os.Getenv("GT_ROOT")
+	defer func() {
+		os.Setenv("GT_TOWN_ROOT", origTownRoot)
+		os.Setenv("GT_ROOT", origRoot)
+	}()
+
+	// Create a temp directory that looks like a valid town
+	tmpTown := t.TempDir()
+	mayorDir := filepath.Join(tmpTown, "mayor")
+	if err := os.MkdirAll(mayorDir, 0755); err != nil {
+		t.Fatalf("creating mayor dir: %v", err)
+	}
+	townJSON := filepath.Join(mayorDir, "town.json")
+	if err := os.WriteFile(townJSON, []byte(`{"name": "test-town"}`), 0644); err != nil {
+		t.Fatalf("creating town.json: %v", err)
+	}
+
+	// Clear both env vars initially
+	os.Setenv("GT_TOWN_ROOT", "")
+	os.Setenv("GT_ROOT", "")
+
+	t.Run("uses GT_TOWN_ROOT when cwd detection fails", func(t *testing.T) {
+		// Set GT_TOWN_ROOT to our temp town
+		os.Setenv("GT_TOWN_ROOT", tmpTown)
+		os.Setenv("GT_ROOT", "")
+
+		// Save cwd, cd to a non-town directory, and restore after
+		origCwd, _ := os.Getwd()
+		os.Chdir(os.TempDir())
+		defer os.Chdir(origCwd)
+
+		result := detectTownRootFromCwd()
+		if result != tmpTown {
+			t.Errorf("detectTownRootFromCwd() = %q, want %q (should use GT_TOWN_ROOT fallback)", result, tmpTown)
+		}
+	})
+
+	t.Run("uses GT_ROOT when GT_TOWN_ROOT not set", func(t *testing.T) {
+		// Set only GT_ROOT
+		os.Setenv("GT_TOWN_ROOT", "")
+		os.Setenv("GT_ROOT", tmpTown)
+
+		// Save cwd, cd to a non-town directory, and restore after
+		origCwd, _ := os.Getwd()
+		os.Chdir(os.TempDir())
+		defer os.Chdir(origCwd)
+
+		result := detectTownRootFromCwd()
+		if result != tmpTown {
+			t.Errorf("detectTownRootFromCwd() = %q, want %q (should use GT_ROOT fallback)", result, tmpTown)
+		}
+	})
+
+	t.Run("prefers GT_TOWN_ROOT over GT_ROOT", func(t *testing.T) {
+		// Create another temp town for GT_ROOT
+		anotherTown := t.TempDir()
+		anotherMayor := filepath.Join(anotherTown, "mayor")
+		os.MkdirAll(anotherMayor, 0755)
+		os.WriteFile(filepath.Join(anotherMayor, "town.json"), []byte(`{"name": "other-town"}`), 0644)
+
+		// Set both env vars
+		os.Setenv("GT_TOWN_ROOT", tmpTown)
+		os.Setenv("GT_ROOT", anotherTown)
+
+		// Save cwd, cd to a non-town directory, and restore after
+		origCwd, _ := os.Getwd()
+		os.Chdir(os.TempDir())
+		defer os.Chdir(origCwd)
+
+		result := detectTownRootFromCwd()
+		if result != tmpTown {
+			t.Errorf("detectTownRootFromCwd() = %q, want %q (should prefer GT_TOWN_ROOT)", result, tmpTown)
+		}
+	})
+
+	t.Run("ignores invalid GT_TOWN_ROOT", func(t *testing.T) {
+		// Set GT_TOWN_ROOT to non-existent path, GT_ROOT to valid
+		os.Setenv("GT_TOWN_ROOT", "/nonexistent/path/to/town")
+		os.Setenv("GT_ROOT", tmpTown)
+
+		// Save cwd, cd to a non-town directory, and restore after
+		origCwd, _ := os.Getwd()
+		os.Chdir(os.TempDir())
+		defer os.Chdir(origCwd)
+
+		result := detectTownRootFromCwd()
+		if result != tmpTown {
+			t.Errorf("detectTownRootFromCwd() = %q, want %q (should skip invalid GT_TOWN_ROOT and use GT_ROOT)", result, tmpTown)
+		}
+	})
+
+	t.Run("uses secondary marker when primary missing", func(t *testing.T) {
+		// Create a temp town with only mayor/ directory (no town.json)
+		secondaryTown := t.TempDir()
+		mayorOnlyDir := filepath.Join(secondaryTown, workspace.SecondaryMarker)
+		os.MkdirAll(mayorOnlyDir, 0755)
+
+		os.Setenv("GT_TOWN_ROOT", secondaryTown)
+		os.Setenv("GT_ROOT", "")
+
+		// Save cwd, cd to a non-town directory, and restore after
+		origCwd, _ := os.Getwd()
+		os.Chdir(os.TempDir())
+		defer os.Chdir(origCwd)
+
+		result := detectTownRootFromCwd()
+		if result != secondaryTown {
+			t.Errorf("detectTownRootFromCwd() = %q, want %q (should accept secondary marker)", result, secondaryTown)
+		}
+	})
+}