feat(export): add --priority exact match filter (bd-au0.6)

Add the --priority (-p) flag to bd export for filtering by exact priority,
matching the behavior of bd list. This completes the comprehensive filtering
support for bd export.

The flag uses cmd.Flags().Changed() to properly handle P0 (priority 0), which
would otherwise be interpreted as "not set" due to Go zero-value semantics.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/export.go

@@ -209,6 +209,17 @@ Examples:
 			filter.LabelsAny = labelsAny
 		}
 
+		// Priority exact match (use Changed() to properly handle P0)
+		if cmd.Flags().Changed("priority") {
+			priorityStr, _ := cmd.Flags().GetString("priority")
+			priority, err := validation.ValidatePriority(priorityStr)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "Error parsing --priority: %v\n", err)
+				os.Exit(1)
+			}
+			filter.Priority = &priority
+		}
+
 		// Priority ranges
 		if cmd.Flags().Changed("priority-min") {
 			priorityMin, err := validation.ValidatePriority(priorityMinStr)
@@ -536,6 +547,7 @@ func init() {
 	exportCmd.Flags().BoolVar(&jsonOutput, "json", false, "Output export statistics in JSON format")
 
 	// Filter flags
+	registerPriorityFlag(exportCmd, "")
 	exportCmd.Flags().StringP("assignee", "a", "", "Filter by assignee")
 	exportCmd.Flags().StringP("type", "t", "", "Filter by type (bug, feature, task, epic, chore, merge-request, molecule, gate)")
 	exportCmd.Flags().StringSliceP("label", "l", []string{}, "Filter by labels (AND: must have ALL)")