feat(blocked): exclude pinned issues from bd blocked output

- Add Pinned field to Issue struct in types.go
- Create migration 023 to add pinned column with partial index
- Update SQLite GetBlockedIssues to filter WHERE pinned = 0
- Update Memory GetBlockedIssues to skip pinned issues
- Update schema.go with pinned column definition

Pinned issues are tracked but excluded from the blocked list to
reduce noise for issues that are intentionally parked.

Closes: beads-ei4

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

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

internal/storage/memory/memory.go

@@ -1063,6 +1063,7 @@ func (m *MemoryStorage) getOpenBlockers(issueID string) []string {
 }
 
 // GetBlockedIssues returns issues that are blocked by other issues
+// Note: Pinned issues are excluded from the output (beads-ei4)
 func (m *MemoryStorage) GetBlockedIssues(ctx context.Context) ([]*types.BlockedIssue, error) {
 	m.mu.RLock()
 	defer m.mu.RUnlock()
@@ -1075,6 +1076,11 @@ func (m *MemoryStorage) GetBlockedIssues(ctx context.Context) ([]*types.BlockedI
 			continue
 		}
 
+		// Exclude pinned issues (beads-ei4)
+		if issue.Pinned {
+			continue
+		}
+
 		blockers := m.getOpenBlockers(issue.ID)
 		if issue.Status != types.StatusBlocked && len(blockers) == 0 {
 			continue