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/clean.go

@@ -13,11 +13,11 @@ import (
 
 var cleanCmd = &cobra.Command{
 	Use:   "clean",
-	Short: "Clean up temporary beads artifacts",
-	Long: `Delete temporary beads artifacts to clean up after git operations.
+	Short: "Clean up temporary git merge artifacts from .beads directory",
+	Long: `Delete temporary git merge artifacts from the .beads directory.
 
-This removes temporary files created during git merges and conflicts from the
-.beads directory.
+This command removes temporary files created during git merges and conflicts.
+It does NOT delete issues from the database - use 'bd cleanup' for that.
 
 Files removed:
 - 3-way merge snapshots (beads.base.jsonl, beads.left.jsonl, beads.right.jsonl)
@@ -36,7 +36,10 @@ Clean up temporary files:
   bd clean
 
 Preview what would be deleted:
-  bd clean --dry-run`,
+  bd clean --dry-run
+
+SEE ALSO:
+  bd cleanup    Delete closed issues from database`,
 	Run: func(cmd *cobra.Command, args []string) {
 		dryRun, _ := cmd.Flags().GetBool("dry-run")