feat(ready): exclude pinned issues from bd ready (beads-92u)

Pinned issues are persistent anchors that should not appear in ready
work lists. This adds:

- Pinned bool field to Issue struct
- pinned INTEGER DEFAULT 0 column to schema
- Migration 023 to add pinned column to existing databases
- WHERE i.pinned = 0 filter in GetReadyWork query

🤖 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.
+
+	// Pinned issues are persistent anchors, excluded from bd ready (bd-92u)
+	Pinned bool `json:"pinned,omitempty"`
 }
 
 // ComputeContentHash creates a deterministic hash of the issue's content.