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>

internal/debug/debug.go

@@ -5,20 +5,54 @@ import (
 	"os"
 )
 
-var enabled = os.Getenv("BD_DEBUG") != ""
+var (
+	enabled     = os.Getenv("BD_DEBUG") != ""
+	verboseMode = false
+	quietMode   = false
+)
 
 func Enabled() bool {
-	return enabled
+	return enabled || verboseMode
+}
+
+// SetVerbose enables verbose/debug output
+func SetVerbose(verbose bool) {
+	verboseMode = verbose
+}
+
+// SetQuiet enables quiet mode (suppress non-essential output)
+func SetQuiet(quiet bool) {
+	quietMode = quiet
+}
+
+// IsQuiet returns true if quiet mode is enabled
+func IsQuiet() bool {
+	return quietMode
 }
 
 func Logf(format string, args ...interface{}) {
-	if enabled {
+	if enabled || verboseMode {
 		fmt.Fprintf(os.Stderr, format, args...)
 	}
 }
 
 func Printf(format string, args ...interface{}) {
-	if enabled {
+	if enabled || verboseMode {
 		fmt.Printf(format, args...)
 	}
 }
+
+// PrintNormal prints output unless quiet mode is enabled
+// Use this for normal informational output that should be suppressed in quiet mode
+func PrintNormal(format string, args ...interface{}) {
+	if !quietMode {
+		fmt.Printf(format, args...)
+	}
+}
+
+// PrintlnNormal prints a line unless quiet mode is enabled
+func PrintlnNormal(args ...interface{}) {
+	if !quietMode {
+		fmt.Println(args...)
+	}
+}