feat(export): add --id and --parent filters (#1292)

Add two new filter flags to the export command:
- --id: Filter by specific issue IDs (comma-separated)
- --parent: Filter by parent issue ID (shows children)

Also fix safety checks (empty DB, staleness) to skip when filters
are active, since filtered exports intentionally produce subsets.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/export.go

@@ -150,6 +150,8 @@ Examples:
 		createdBefore, _ := cmd.Flags().GetString("created-before")
 		updatedAfter, _ := cmd.Flags().GetString("updated-after")
 		updatedBefore, _ := cmd.Flags().GetString("updated-before")
+		idFilter, _ := cmd.Flags().GetString("id")
+		parentID, _ := cmd.Flags().GetString("parent")
 
 		debug.Logf("Debug: export flags - output=%q, force=%v\n", output, force)
 
@@ -225,6 +227,15 @@ Examples:
 		if len(labelsAny) > 0 {
 			filter.LabelsAny = labelsAny
 		}
+		if idFilter != "" {
+			ids := util.NormalizeLabels(strings.Split(idFilter, ","))
+			if len(ids) > 0 {
+				filter.IDs = ids
+			}
+		}
+		if parentID != "" {
+			filter.ParentID = &parentID
+		}
 
 		// Priority exact match (use Changed() to properly handle P0)
 		if cmd.Flags().Changed("priority") {
@@ -297,8 +308,27 @@ Examples:
 			os.Exit(1)
 		}
 
+		// Detect if any substantive filters are active (partial export)
+		// Safety checks should be skipped for filtered exports since the user
+		// intentionally wants a subset of issues, not the full database
+		isFilteredExport := filter.Status != nil ||
+			filter.Assignee != nil ||
+			filter.IssueType != nil ||
+			len(filter.Labels) > 0 ||
+			len(filter.LabelsAny) > 0 ||
+			len(filter.IDs) > 0 ||
+			filter.ParentID != nil ||
+			filter.Priority != nil ||
+			filter.PriorityMin != nil ||
+			filter.PriorityMax != nil ||
+			filter.CreatedAfter != nil ||
+			filter.CreatedBefore != nil ||
+			filter.UpdatedAfter != nil ||
+			filter.UpdatedBefore != nil
+
 		// Safety check: prevent exporting empty database over non-empty JSONL
-		if len(issues) == 0 && output != "" && !force {
+		// Skip this check for filtered exports - the user intentionally wants a subset
+		if len(issues) == 0 && output != "" && !force && !isFilteredExport {
 			existingCount, err := countIssuesInJSONL(output)
 			if err != nil {
 				// If we can't read the file, it might not exist yet, which is fine
@@ -317,7 +347,8 @@ Examples:
 		}
 
 		// Safety check: prevent exporting stale database that would lose issues
-		if output != "" && !force {
+		// Skip this check for filtered exports - the user intentionally wants a subset
+		if output != "" && !force && !isFilteredExport {
 			debug.Logf("Debug: checking staleness - output=%s, force=%v\n", output, force)
 
 			// Read existing JSONL to get issue IDs
@@ -616,5 +647,11 @@ func init() {
 	exportCmd.Flags().String("updated-after", "", "Filter issues updated after date (YYYY-MM-DD or RFC3339)")
 	exportCmd.Flags().String("updated-before", "", "Filter issues updated before date (YYYY-MM-DD or RFC3339)")
 
+	// ID filter
+	exportCmd.Flags().String("id", "", "Filter by specific issue IDs (comma-separated, e.g., bd-1,bd-5,bd-10)")
+
+	// Parent filter
+	exportCmd.Flags().String("parent", "", "Filter by parent issue ID (shows children of specified issue)")
+
 	rootCmd.AddCommand(exportCmd)
 }