Add no-db as a persistent configuration option

This allows users to set --no-db mode persistently via:
1. .beads/config.yaml file (no-db: true)
2. BD_NO_DB environment variable
3. --no-db command-line flag (highest precedence)

Changes:
- Add no-db to config defaults in internal/config/config.go
- Wire no-db flag to read from config in cmd/bd/main.go
- Create example .beads/config.yaml with documentation

The configuration precedence is:
  CLI flag > Environment variable > Config file > Default

This makes no-db mode repository-scoped and automatically
respected by all bd commands and the beads-mcp service.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

cmd/bd/main.go

@@ -105,6 +105,9 @@ var rootCmd = &cobra.Command{
 		if !cmd.Flags().Changed("no-auto-import") {
 			noAutoImport = config.GetBool("no-auto-import")
 		}
+		if !cmd.Flags().Changed("no-db") {
+			noDb = config.GetBool("no-db")
+		}
 		if !cmd.Flags().Changed("db") && dbPath == "" {
 			dbPath = config.GetString("db")
 		}