fix: validate project files in FindBeadsDir (bd-420) (#424)

FindBeadsDir() now checks for actual beads project files before returning
a .beads directory. This prevents false positives when ~/.beads/ exists
only for daemon registry (registry.json).

Changes:
- Add hasBeadsProjectFiles() helper that checks for:
  - metadata.json or config.yaml (project config)
  - *.db files (excluding backups and vc.db)
  - *.jsonl files (JSONL-only mode)
- Update FindBeadsDir() to validate directories during tree search
- Add comprehensive tests for project file detection
- Update version_tracking_test.go to create project files

Fixes #420

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

cmd/bd/version_tracking_test.go

@@ -124,12 +124,18 @@ func TestTrackBdVersion_NoBeadsDir(t *testing.T) {
 }
 
 func TestTrackBdVersion_FirstRun(t *testing.T) {
-	// Create temp .beads directory
+	// Create temp .beads directory with a project file (bd-420)
+	// FindBeadsDir now requires actual project files, not just directory existence
 	tmpDir := t.TempDir()
 	beadsDir := filepath.Join(tmpDir, ".beads")
 	if err := os.MkdirAll(beadsDir, 0755); err != nil {
 		t.Fatalf("Failed to create .beads: %v", err)
 	}
+	// Create a database file so FindBeadsDir finds this directory
+	dbPath := filepath.Join(beadsDir, "beads.db")
+	if err := os.WriteFile(dbPath, []byte{}, 0644); err != nil {
+		t.Fatalf("Failed to create db file: %v", err)
+	}
 
 	// Change to temp directory
 	origWd, _ := os.Getwd()