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

cmd/bd/init.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/fatih/color"
 	"github.com/spf13/cobra"
+	"github.com/steveyegge/beads"
 	"github.com/steveyegge/beads/internal/configfile"
 	"github.com/steveyegge/beads/internal/storage/sqlite"
 )
@@ -48,7 +49,7 @@ and database file. Optionally specify a custom issue prefix.`,
 		// Use global dbPath if set via --db flag or BEADS_DB env var, otherwise default to .beads/beads.db
 		initDBPath := dbPath
 		if initDBPath == "" {
-		initDBPath = filepath.Join(".beads", "beads.db")
+		initDBPath = filepath.Join(".beads", beads.CanonicalDatabaseName)
 		}
 
 		// Migrate old database files if they exist