Add warning for multiple databases in directory hierarchy (bd-75)

- Implement FindAllDatabases() to scan hierarchy for all .beads directories
- Add DatabaseInfo struct with path, beads dir, and issue count
- Add warnMultipleDatabases() with formatted warning display
- Show active database with ▶ marker and issue counts
- Add comprehensive tests for multi-database detection
- Document warning and solutions in TROUBLESHOOTING.md
- Prevent confusion and database pollution from accidental duplicates

Amp-Thread-ID: https://ampcode.com/threads/T-4941975f-2686-40d0-bc12-aabf38a05890
Co-authored-by: Amp <amp@ampcode.com>

beads_multidb_test.go

@@ -0,0 +1,168 @@
+package beads
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestFindAllDatabases(t *testing.T) {
+	// Create a temporary directory structure with multiple .beads databases
+	tmpDir, err := os.MkdirTemp("", "beads-multidb-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// Resolve symlinks (macOS /var -> /private/var)
+	tmpDir, err = filepath.EvalSymlinks(tmpDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create nested directory structure:
+	// tmpDir/
+	//   .beads/test.db
+	//   project1/
+	//     .beads/project1.db
+	//     subdir/
+	//       (working directory here)
+
+	// Root .beads
+	rootBeads := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(rootBeads, 0755); err != nil {
+		t.Fatal(err)
+	}
+	rootDB := filepath.Join(rootBeads, "test.db")
+	if err := os.WriteFile(rootDB, []byte("fake db"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Project1 .beads
+	project1Dir := filepath.Join(tmpDir, "project1")
+	project1Beads := filepath.Join(project1Dir, ".beads")
+	if err := os.MkdirAll(project1Beads, 0755); err != nil {
+		t.Fatal(err)
+	}
+	project1DB := filepath.Join(project1Beads, "project1.db")
+	if err := os.WriteFile(project1DB, []byte("fake db"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Subdir for working directory
+	subdir := filepath.Join(project1Dir, "subdir")
+	if err := os.MkdirAll(subdir, 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	// Save original working directory
+	origDir, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Chdir(origDir)
+
+	// Change to subdir and test FindAllDatabases
+	if err := os.Chdir(subdir); err != nil {
+		t.Fatal(err)
+	}
+
+	databases := FindAllDatabases()
+
+	// Should find both databases, with project1 first (closest)
+	if len(databases) != 2 {
+		t.Fatalf("expected 2 databases, got %d", len(databases))
+	}
+
+	// First database should be project1 (closest to CWD)
+	if databases[0].Path != project1DB {
+		t.Errorf("expected first database to be %s, got %s", project1DB, databases[0].Path)
+	}
+	if databases[0].BeadsDir != project1Beads {
+		t.Errorf("expected first beads dir to be %s, got %s", project1Beads, databases[0].BeadsDir)
+	}
+
+	// Second database should be root (furthest from CWD)
+	if databases[1].Path != rootDB {
+		t.Errorf("expected second database to be %s, got %s", rootDB, databases[1].Path)
+	}
+	if databases[1].BeadsDir != rootBeads {
+		t.Errorf("expected second beads dir to be %s, got %s", rootBeads, databases[1].BeadsDir)
+	}
+}
+
+func TestFindAllDatabases_Single(t *testing.T) {
+	// Create a temporary directory with only one database
+	tmpDir, err := os.MkdirTemp("", "beads-single-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// Resolve symlinks (macOS /var -> /private/var)
+	tmpDir, err = filepath.EvalSymlinks(tmpDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create .beads directory with database
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		t.Fatal(err)
+	}
+	dbPath := filepath.Join(beadsDir, "test.db")
+	if err := os.WriteFile(dbPath, []byte("fake db"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	// Save original working directory
+	origDir, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Chdir(origDir)
+
+	// Change to tmpDir and test
+	if err := os.Chdir(tmpDir); err != nil {
+		t.Fatal(err)
+	}
+
+	databases := FindAllDatabases()
+
+	// Should find exactly one database
+	if len(databases) != 1 {
+		t.Fatalf("expected 1 database, got %d", len(databases))
+	}
+
+	if databases[0].Path != dbPath {
+		t.Errorf("expected database path %s, got %s", dbPath, databases[0].Path)
+	}
+}
+
+func TestFindAllDatabases_None(t *testing.T) {
+	// Create a temporary directory with no databases
+	tmpDir, err := os.MkdirTemp("", "beads-none-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// Save original working directory
+	origDir, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Chdir(origDir)
+
+	// Change to tmpDir and test
+	if err := os.Chdir(tmpDir); err != nil {
+		t.Fatal(err)
+	}
+
+	databases := FindAllDatabases()
+
+	// Should find no databases
+	if len(databases) != 0 {
+		t.Fatalf("expected 0 databases, got %d", len(databases))
+	}
+}