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/rename_prefix.go

@@ -18,10 +18,16 @@ import (
 
 var renamePrefixCmd = &cobra.Command{
 	Use:   "rename-prefix <new-prefix>",
-	Short: "Rename the issue prefix for all issues",
+	Short: "Rename the issue prefix for all issues in the database",
 	Long: `Rename the issue prefix for all issues in the database.
 This will update all issue IDs and all text references across all fields.
 
+USE CASES:
+- Shortening long prefixes (e.g., 'knowledge-work-' → 'kw-')
+- Rebranding project naming conventions
+- Consolidating multiple prefixes after database corruption
+- Migrating to team naming standards
+
 Prefix validation rules:
 - Max length: 8 characters
 - Allowed characters: lowercase letters, numbers, hyphens
@@ -34,9 +40,12 @@ If issues have multiple prefixes (corrupted database), use --repair to consolida
 The --repair flag will rename all issues with incorrect prefixes to the new prefix,
 preserving issues that already have the correct prefix.
 
-Example:
-  bd rename-prefix kw-         # Rename from 'knowledge-work-' to 'kw-'
-  bd rename-prefix mtg- --repair  # Consolidate multiple prefixes into 'mtg-'`,
+EXAMPLES:
+  bd rename-prefix kw-                # Rename from 'knowledge-work-' to 'kw-'
+  bd rename-prefix mtg- --repair      # Consolidate multiple prefixes into 'mtg-'
+  bd rename-prefix team- --dry-run    # Preview changes without applying
+
+NOTE: This is a rare operation. Most users never need this command.`,
 	Args: cobra.ExactArgs(1),
 	Run: func(cmd *cobra.Command, args []string) {
 		newPrefix := args[0]