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>

cmd/bd/main.go

@@ -394,6 +394,9 @@ var rootCmd = &cobra.Command{
 		storeActive = true
 		storeMutex.Unlock()
 
+		// Warn if multiple databases detected in directory hierarchy
+		warnMultipleDatabases(dbPath)
+
 		// Check for version mismatch (warn if binary is older than DB)
 		checkVersionMismatch()