feat: Complete command set standardization (bd-au0)

Epic bd-au0: Command Set Standardization & Flag Consistency

Completed all 10 child issues:

P0 tasks:
- Standardize --dry-run flag across all commands (bd-au0.1)
- Add label operations to bd update (bd-au0.2)
- Fix --title vs --title-contains redundancy (bd-au0.3)
- Standardize priority flag parsing (bd-au0.4)

P1 tasks:
- Add date/priority filters to bd search (bd-au0.5)
- Add comprehensive filters to bd export (bd-au0.6)
- Audit and standardize JSON output (bd-au0.7)

P2 tasks:
- Improve clean vs cleanup documentation (bd-au0.8)
- Document rarely-used commands (bd-au0.9)

P3 tasks:
- Add global verbosity flags --verbose/-v and --quiet/-q (bd-au0.10)

Key changes:
- export.go: Added filters (assignee, type, labels, priority, dates)
- main.go: Added --verbose/-v and --quiet/-q global flags
- debug.go: Added SetVerbose/SetQuiet and PrintNormal helpers
- clean.go/cleanup.go: Improved documentation with cross-references
- detect_pollution.go: Added use cases and warnings
- migrate_hash_ids.go: Marked as legacy command
- rename_prefix.go: Added use cases documentation

All success criteria met: flags standardized, feature parity achieved,
naming clarified, JSON output consistent.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/main.go

@@ -99,6 +99,8 @@ var (
 	profileEnabled bool
 	profileFile    *os.File
 	traceFile      *os.File
+	verboseFlag    bool // Enable verbose/debug output
+	quietFlag      bool // Suppress non-essential output
 )
 
 func init() {
@@ -118,9 +120,11 @@ func init() {
 	rootCmd.PersistentFlags().BoolVar(&allowStale, "allow-stale", false, "Allow operations on potentially stale data (skip staleness check)")
 	rootCmd.PersistentFlags().BoolVar(&noDb, "no-db", false, "Use no-db mode: load from JSONL, no SQLite")
 	rootCmd.PersistentFlags().BoolVar(&profileEnabled, "profile", false, "Generate CPU profile for performance analysis")
+	rootCmd.PersistentFlags().BoolVarP(&verboseFlag, "verbose", "v", false, "Enable verbose/debug output")
+	rootCmd.PersistentFlags().BoolVarP(&quietFlag, "quiet", "q", false, "Suppress non-essential output (errors only)")
 
 	// Add --version flag to root command (same behavior as version subcommand)
-	rootCmd.Flags().BoolP("version", "v", false, "Print version information")
+	rootCmd.Flags().BoolP("version", "V", false, "Print version information")
 }
 
 var rootCmd = &cobra.Command{
@@ -140,6 +144,10 @@ var rootCmd = &cobra.Command{
 		// Set up signal-aware context for graceful cancellation
 		rootCtx, rootCancel = signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 
+		// Apply verbosity flags early (before any output)
+		debug.SetVerbose(verboseFlag)
+		debug.SetQuiet(quietFlag)
+
 		// Apply viper configuration if flags weren't explicitly set
 		// Priority: flags > viper (config file + env vars) > defaults
 		// Do this BEFORE early-return so init/version/help respect config