feat(storage): add pinned field to issues schema

Add pinned column to the issues table to support persistent context markers
that should not be treated as work items (bd-7h5).

Changes:
- Add pinned column to schema.go CREATE TABLE
- Add migration 023_pinned_column.go for existing databases
- Update all issue queries to include pinned column
- Update scanIssues and scanIssuesWithDependencyType to handle pinned field
- Add Pinned field to types.Issue struct with JSON serialization
- Fix migrations_test.go to include pinned in legacy schema test

🤖 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 persistent context markers that should not be treated as work items (bd-7h5).
+func MigratePinnedColumn(db *sql.DB) error {
+	// Check if column already 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 the pinned column
+	_, 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 index for pinned issues (for efficient filtering)
+	_, 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
+}