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/sqlite/migrations.go

@@ -39,6 +39,7 @@ var migrationsList = []Migration{
 	{"edge_consolidation", migrations.MigrateEdgeConsolidation},
 	{"migrate_edge_fields", migrations.MigrateEdgeFields},
 	{"drop_edge_columns", migrations.MigrateDropEdgeColumns},
+	{"pinned_column", migrations.MigratePinnedColumn},
 }
 
 // MigrationInfo contains metadata about a migration for inspection
@@ -85,6 +86,7 @@ func getMigrationDescription(name string) string {
 		"edge_consolidation":           "Adds metadata and thread_id columns to dependencies table for edge schema consolidation (Decision 004)",
 		"migrate_edge_fields":          "Migrates existing issue fields (replies_to, relates_to, duplicate_of, superseded_by) to dependency edges (Decision 004 Phase 3)",
 		"drop_edge_columns":            "Drops deprecated edge columns (replies_to, relates_to, duplicate_of, superseded_by) from issues table (Decision 004 Phase 4)",
+		"pinned_column":                "Adds pinned column to issues table for excluding issues from bd blocked output (beads-ei4)",
 	}
 	
 	if desc, ok := descriptions[name]; ok {