fix: add missing created_by column to migration 028 (GH#757)

Migration 028 (tombstone_closed_at) recreates the issues table but was
missing the created_by column that exists in schema.go. This caused
`SELECT * FROM issues` to fail with column count mismatch on fresh DBs.

🤖 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

@@ -48,6 +48,7 @@ func MigrateTombstoneClosedAt(db *sql.DB) error {
 			assignee TEXT,
 			estimated_minutes INTEGER,
 			created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+			created_by TEXT DEFAULT '',
 			updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
 			closed_at DATETIME,
 			external_ref TEXT,