fix: add idempotency check to migration 028 (tombstone_closed_at)

Migration 028 lacked an idempotency check, causing it to fail on databases
where the migration had already been applied. The migration would attempt
to copy data from issues to issues_new, but both tables had the same CHECK
constraint, causing the insert to fail.

Added check for "status = 'tombstone'" in the table schema to detect if
the migration has already been applied and skip if so.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/migrations/028_tombstone_closed_at.go

@@ -3,6 +3,7 @@ package migrations
 import (
 	"database/sql"
 	"fmt"
+	"strings"
 )
 
 // MigrateTombstoneClosedAt updates the closed_at constraint to allow tombstones
@@ -22,8 +23,20 @@ func MigrateTombstoneClosedAt(db *sql.DB) error {
 	// SQLite doesn't support ALTER TABLE to modify CHECK constraints
 	// We must recreate the table with the new constraint
 
+	// Idempotency check: see if the new CHECK constraint already exists
+	// The new constraint contains "status = 'tombstone'" which the old one didn't
+	var tableSql string
+	err := db.QueryRow(`SELECT sql FROM sqlite_master WHERE type='table' AND name='issues'`).Scan(&tableSql)
+	if err != nil {
+		return fmt.Errorf("failed to get issues table schema: %w", err)
+	}
+	// If the schema already has the tombstone clause, migration is already applied
+	if strings.Contains(tableSql, "status = 'tombstone'") || strings.Contains(tableSql, `status = "tombstone"`) {
+		return nil
+	}
+
 	// Step 0: Drop views that depend on the issues table
-	_, err := db.Exec(`DROP VIEW IF EXISTS ready_issues`)
+	_, err = db.Exec(`DROP VIEW IF EXISTS ready_issues`)
 	if err != nil {
 		return fmt.Errorf("failed to drop ready_issues view: %w", err)
 	}