Add --clear-duplicate-external-refs flag to bd import

Fixes GH-234 by providing automatic resolution for duplicate external_ref
values instead of forcing manual JSONL editing.

Changes:
- Add ClearDuplicateExternalRefs option to importer.Options
- Modify validateNoDuplicateExternalRefs to clear duplicates when enabled
- Keep first occurrence, clear rest when flag is set
- Enhanced error message to suggest the flag
- Add comprehensive tests for the new behavior

Usage: bd import -i issues.jsonl --clear-duplicate-external-refs
Amp-Thread-ID: https://ampcode.com/threads/T-932dcf45-76f2-4994-9b5c-a6eb20a86036
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/import.go

@@ -33,6 +33,7 @@ Behavior:
 		dryRun, _ := cmd.Flags().GetBool("dry-run")
 		renameOnImport, _ := cmd.Flags().GetBool("rename-on-import")
 		dedupeAfter, _ := cmd.Flags().GetBool("dedupe-after")
+		clearDuplicateExternalRefs, _ := cmd.Flags().GetBool("clear-duplicate-external-refs")
 		orphanHandling, _ := cmd.Flags().GetString("orphan-handling")
 
 		// Open input
@@ -95,11 +96,12 @@ Behavior:
 
 		// Phase 2: Use shared import logic
 		opts := ImportOptions{
-			DryRun:         dryRun,
-			SkipUpdate:     skipUpdate,
-			Strict:         strict,
-			RenameOnImport: renameOnImport,
-			OrphanHandling: orphanHandling,
+			DryRun:                     dryRun,
+			SkipUpdate:                 skipUpdate,
+			Strict:                     strict,
+			RenameOnImport:             renameOnImport,
+			ClearDuplicateExternalRefs: clearDuplicateExternalRefs,
+			OrphanHandling:             orphanHandling,
 		}
 
 		result, err := importIssuesCore(ctx, dbPath, store, allIssues, opts)
@@ -265,6 +267,7 @@ func init() {
 	importCmd.Flags().Bool("dedupe-after", false, "Detect and report content duplicates after import")
 	importCmd.Flags().Bool("dry-run", false, "Preview collision detection without making changes")
 	importCmd.Flags().Bool("rename-on-import", false, "Rename imported issues to match database prefix (updates all references)")
+	importCmd.Flags().Bool("clear-duplicate-external-refs", false, "Clear duplicate external_ref values (keeps first occurrence)")
 	importCmd.Flags().String("orphan-handling", "", "How to handle missing parent issues: strict/resurrect/skip/allow (default: use config or 'allow')")
 	importCmd.Flags().BoolVar(&jsonOutput, "json", false, "Output import statistics in JSON format")
 	rootCmd.AddCommand(importCmd)

cmd/bd/import_shared.go

@@ -158,12 +158,13 @@ func issueDataChanged(existing *types.Issue, updates map[string]interface{}) boo
 
 // ImportOptions configures how the import behaves
 type ImportOptions struct {
-	DryRun             bool   // Preview changes without applying them
-	SkipUpdate         bool   // Skip updating existing issues (create-only mode)
-	Strict             bool   // Fail on any error (dependencies, labels, etc.)
-	RenameOnImport     bool   // Rename imported issues to match database prefix
-	SkipPrefixValidation bool // Skip prefix validation (for auto-import)
-	OrphanHandling     string // Orphan handling mode: strict/resurrect/skip/allow (empty = use config)
+	DryRun                     bool   // Preview changes without applying them
+	SkipUpdate                 bool   // Skip updating existing issues (create-only mode)
+	Strict                     bool   // Fail on any error (dependencies, labels, etc.)
+	RenameOnImport             bool   // Rename imported issues to match database prefix
+	SkipPrefixValidation       bool   // Skip prefix validation (for auto-import)
+	ClearDuplicateExternalRefs bool   // Clear duplicate external_ref values instead of erroring
+	OrphanHandling             string // Orphan handling mode: strict/resurrect/skip/allow (empty = use config)
 }
 
 // ImportResult contains statistics about the import operation
@@ -210,12 +211,13 @@ func importIssuesCore(ctx context.Context, dbPath string, store storage.Storage,
 	
 	// Convert ImportOptions to importer.Options
 	importerOpts := importer.Options{
-		DryRun:               opts.DryRun,
-		SkipUpdate:           opts.SkipUpdate,
-		Strict:               opts.Strict,
-		RenameOnImport:       opts.RenameOnImport,
-		SkipPrefixValidation: opts.SkipPrefixValidation,
-		OrphanHandling:       importer.OrphanHandling(orphanHandling),
+		DryRun:                     opts.DryRun,
+		SkipUpdate:                 opts.SkipUpdate,
+		Strict:                     opts.Strict,
+		RenameOnImport:             opts.RenameOnImport,
+		SkipPrefixValidation:       opts.SkipPrefixValidation,
+		ClearDuplicateExternalRefs: opts.ClearDuplicateExternalRefs,
+		OrphanHandling:             importer.OrphanHandling(orphanHandling),
 	}
 
 	// Delegate to the importer package