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/types/types.go

@@ -45,6 +45,9 @@ type Issue struct {
 	Ephemeral bool   `json:"ephemeral,omitempty"` // Can be bulk-deleted when closed
 	// NOTE: RepliesTo, RelatesTo, DuplicateOf, SupersededBy moved to dependencies table
 	// per Decision 004 (Edge Schema Consolidation). Use dependency API instead.
+
+	// Workflow fields
+	Pinned bool `json:"pinned,omitempty"` // Pinned issues are excluded from bd blocked output
 }
 
 // ComputeContentHash creates a deterministic hash of the issue's content.