Enforce canonical database naming (beads.db) - bd-165

- Added CanonicalDatabaseName constant (beads.db) and LegacyDatabaseNames list
- Updated bd init to use canonical name via constant
- Added daemon validation to reject non-canonical database names
- Updated bd migrate to use canonical name constant
- Enhanced FindDatabasePath to warn when using legacy database names
- All database discovery now prefers beads.db with backward compatibility

Closes bd-165

beads.go

@@ -19,6 +19,12 @@ import (
 	"github.com/steveyegge/beads/internal/types"
 )
 
+// CanonicalDatabaseName is the required database filename for all beads repositories
+const CanonicalDatabaseName = "beads.db"
+
+// LegacyDatabaseNames are old names that should be migrated
+var LegacyDatabaseNames = []string{"bd.db", "issues.db", "bugs.db"}
+
 // Issue represents a tracked work item with metadata, dependencies, and status.
 type (
 	Issue = types.Issue
@@ -184,38 +190,53 @@ func findDatabaseInTree() string {
 			}
 
 			// Fall back to canonical beads.db for backward compatibility
-			canonicalDB := filepath.Join(beadsDir, "beads.db")
+			canonicalDB := filepath.Join(beadsDir, CanonicalDatabaseName)
 			if _, err := os.Stat(canonicalDB); err == nil {
-				return canonicalDB
+			return canonicalDB
 			}
 
 			// Found .beads/ directory, look for *.db files
 			matches, err := filepath.Glob(filepath.Join(beadsDir, "*.db"))
 			if err == nil && len(matches) > 0 {
-				// Filter out backup files
-				var validDBs []string
-				for _, match := range matches {
-					baseName := filepath.Base(match)
-					// Skip backup files (e.g., beads.db.backup, bd.db.backup)
-					if filepath.Ext(baseName) != ".backup" {
-						validDBs = append(validDBs, match)
-					}
-				}
-
-				if len(validDBs) > 1 {
-					// Multiple databases found - this is ambiguous
-					// Print error to stderr but return the first one for backward compatibility
-					fmt.Fprintf(os.Stderr, "Warning: Multiple database files found in %s:\n", beadsDir)
-					for _, db := range validDBs {
-						fmt.Fprintf(os.Stderr, "  - %s\n", filepath.Base(db))
-					}
-					fmt.Fprintf(os.Stderr, "Run 'bd init' to migrate to beads.db or manually remove old databases.\n\n")
-				}
-
-				if len(validDBs) > 0 {
-					return validDBs[0]
-				}
+			// Filter out backup files
+			var validDBs []string
+			for _, match := range matches {
+			baseName := filepath.Base(match)
+			// Skip backup files (e.g., beads.db.backup, bd.db.backup)
+			if filepath.Ext(baseName) != ".backup" {
+			validDBs = append(validDBs, match)
 			}
+			}
+
+			if len(validDBs) > 1 {
+			// Multiple databases found - this is ambiguous
+			// Print error to stderr but return the first one for backward compatibility
+			fmt.Fprintf(os.Stderr, "Warning: Multiple database files found in %s:\n", beadsDir)
+			for _, db := range validDBs {
+			fmt.Fprintf(os.Stderr, "  - %s\n", filepath.Base(db))
+			}
+			fmt.Fprintf(os.Stderr, "Run 'bd init' to migrate to %s or manually remove old databases.\n\n", CanonicalDatabaseName)
+			}
+
+			if len(validDBs) > 0 {
+			// Check if using legacy name and warn
+			 dbName := filepath.Base(validDBs[0])
+			  if dbName != CanonicalDatabaseName {
+					isLegacy := false
+					for _, legacy := range LegacyDatabaseNames {
+						if dbName == legacy {
+							isLegacy = true
+							break
+						}
+					}
+					if isLegacy {
+						fmt.Fprintf(os.Stderr, "WARNING: Using legacy database name: %s\n", dbName)
+						fmt.Fprintf(os.Stderr, "Run 'bd migrate' to upgrade to canonical name: %s\n\n", CanonicalDatabaseName)
+					}
+				}
+				return validDBs[0]
+			}
+		}
 		}
 
 		// Move up one directory