fix: crew start rig inference + refactor overlay to shared utility

Two improvements:

1. gt crew start now infers rig from cwd when first arg is not a valid
   rig name (gt-czltv). Previously, running `gt crew start bob` from
   within a rig directory would fail because "bob" was treated as the
   rig name. Now it checks if the arg is a valid rig first.

2. Refactored copyOverlay to shared rig.CopyOverlay utility:
   - Eliminates code duplication between crew and polecat managers
   - Preserves source file permissions instead of hardcoding 0644
   - Follows PR #278 improvements

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

internal/rig/overlay.go

@@ -0,0 +1,86 @@
+package rig
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+)
+
+// CopyOverlay copies files from <rigPath>/.runtime/overlay/ to the destination path.
+// This allows storing gitignored files (like .env) that services need at their root.
+// The overlay is copied non-recursively - only files, not subdirectories.
+// File permissions from the source are preserved.
+//
+// Structure:
+//
+//	rig/
+//	  .runtime/
+//	    overlay/
+//	      .env          <- Copied to destPath
+//	      config.json   <- Copied to destPath
+//
+// Returns nil if the overlay directory doesn't exist (nothing to copy).
+// Individual file copy failures are logged as warnings but don't stop the process.
+func CopyOverlay(rigPath, destPath string) error {
+	overlayDir := filepath.Join(rigPath, ".runtime", "overlay")
+
+	// Check if overlay directory exists
+	entries, err := os.ReadDir(overlayDir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			// No overlay directory - not an error, just nothing to copy
+			return nil
+		}
+		return fmt.Errorf("reading overlay dir: %w", err)
+	}
+
+	// Copy each file (not directories) from overlay to destination
+	for _, entry := range entries {
+		if entry.IsDir() {
+			// Skip subdirectories - only copy files at overlay root
+			continue
+		}
+
+		srcPath := filepath.Join(overlayDir, entry.Name())
+		dstPath := filepath.Join(destPath, entry.Name())
+
+		if err := copyFilePreserveMode(srcPath, dstPath); err != nil {
+			// Log warning but continue - don't fail spawn for overlay issues
+			fmt.Printf("Warning: could not copy overlay file %s: %v\n", entry.Name(), err)
+			continue
+		}
+	}
+
+	return nil
+}
+
+// copyFilePreserveMode copies a file from src to dst, preserving the source file's permissions.
+func copyFilePreserveMode(src, dst string) error {
+	// Get source file info for permissions
+	srcInfo, err := os.Stat(src)
+	if err != nil {
+		return fmt.Errorf("stat source: %w", err)
+	}
+
+	// Open source file
+	srcFile, err := os.Open(src)
+	if err != nil {
+		return fmt.Errorf("open source: %w", err)
+	}
+	defer srcFile.Close()
+
+	// Create destination file with same permissions
+	dstFile, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, srcInfo.Mode().Perm())
+	if err != nil {
+		return fmt.Errorf("create destination: %w", err)
+	}
+	defer dstFile.Close()
+
+	// Copy contents
+	if _, err := io.Copy(dstFile, srcFile); err != nil {
+		return fmt.Errorf("copy contents: %w", err)
+	}
+
+	return nil
+}