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/migrate_hash_ids.go

@@ -22,17 +22,30 @@ import (
 
 var migrateHashIDsCmd = &cobra.Command{
 	Use:   "migrate-hash-ids",
-	Short: "Migrate sequential IDs to hash-based IDs",
+	Short: "Migrate sequential IDs to hash-based IDs (legacy)",
 	Long: `Migrate database from sequential IDs (bd-1, bd-2) to hash-based IDs (bd-a3f8e9a2).
 
-This command:
+*** LEGACY COMMAND ***
+This is a one-time migration command. Most users do not need this.
+Only use if migrating from an older beads version that used sequential IDs.
+
+What this does:
 - Generates hash IDs for all top-level issues
 - Assigns hierarchical child IDs (bd-a3f8e9a2.1) for epic children
 - Updates all references (dependencies, comments, external refs)
 - Creates mapping file for reference
 - Validates all relationships are intact
+- Automatically creates database backup before migration
 
-Use --dry-run to preview changes before applying.`,
+USE CASES:
+- Upgrading from beads v1.x to v2.x (sequential → hash IDs)
+- One-time migration only - do not run on already-migrated databases
+
+EXAMPLES:
+  bd migrate-hash-ids --dry-run       # Preview changes
+  bd migrate-hash-ids                 # Perform migration (creates backup)
+
+WARNING: Backup your database before running this command, even though it creates one automatically.`,
 	Run: func(cmd *cobra.Command, _ []string) {
 		dryRun, _ := cmd.Flags().GetBool("dry-run")