feat(types): add StatusPinned for persistent beads (bd-6v2)

Add pinned status for beads that should stay open indefinitely:
- Add StatusPinned constant and update IsValid()
- Add PinnedIssues count to Statistics struct
- Protect pinned issues from bd close (requires --force)
- Show pinned count in bd stats output

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

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

cmd/bd/ready.go

@@ -272,6 +272,9 @@ var statsCmd = &cobra.Command{
 			if stats.TombstoneIssues > 0 {
 				fmt.Printf("Deleted:           %d (tombstones)\n", stats.TombstoneIssues)
 			}
+			if stats.PinnedIssues > 0 {
+				fmt.Printf("Pinned:            %d\n", stats.PinnedIssues)
+			}
 			if stats.AverageLeadTime > 0 {
 				fmt.Printf("Avg Lead Time:     %.1f hours\n", stats.AverageLeadTime)
 			}
@@ -313,6 +316,9 @@ var statsCmd = &cobra.Command{
 		if stats.TombstoneIssues > 0 {
 			fmt.Printf("Deleted:                %d (tombstones)\n", stats.TombstoneIssues)
 		}
+		if stats.PinnedIssues > 0 {
+			fmt.Printf("Pinned:                 %d\n", stats.PinnedIssues)
+		}
 		if stats.EpicsEligibleForClosure > 0 {
 			fmt.Printf("Epics Ready to Close:   %s\n", green(fmt.Sprintf("%d", stats.EpicsEligibleForClosure)))
 		}