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/023_pinned_column.go

@@ -0,0 +1,39 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigratePinnedColumn adds the pinned column to the issues table.
+// Pinned issues are excluded from bd blocked output (beads-ei4).
+func MigratePinnedColumn(db *sql.DB) error {
+	// Check if column exists
+	var columnExists bool
+	err := db.QueryRow(`
+		SELECT COUNT(*) > 0
+		FROM pragma_table_info('issues')
+		WHERE name = 'pinned'
+	`).Scan(&columnExists)
+	if err != nil {
+		return fmt.Errorf("failed to check pinned column: %w", err)
+	}
+
+	if columnExists {
+		return nil
+	}
+
+	// Add pinned column (default false = 0)
+	_, err = db.Exec(`ALTER TABLE issues ADD COLUMN pinned INTEGER DEFAULT 0`)
+	if err != nil {
+		return fmt.Errorf("failed to add pinned column: %w", err)
+	}
+
+	// Add partial index for efficient queries on pinned issues
+	_, err = db.Exec(`CREATE INDEX IF NOT EXISTS idx_issues_pinned ON issues(pinned) WHERE pinned = 1`)
+	if err != nil {
+		return fmt.Errorf("failed to create pinned index: %w", err)
+	}
+
+	return nil
+}