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/storage/sqlite/schema.go

@@ -32,6 +32,8 @@ CREATE TABLE IF NOT EXISTS issues (
     ephemeral INTEGER DEFAULT 0,
     -- NOTE: replies_to, relates_to, duplicate_of, superseded_by removed per Decision 004
     -- These relationships are now stored in the dependencies table
+    -- Pinned issues are persistent anchors, excluded from bd ready (bd-92u)
+    pinned INTEGER DEFAULT 0,
     CHECK ((status = 'closed') = (closed_at IS NOT NULL))
 );