fix(runtime-host): drop unread scheduled-task backend persistence

[c020627]

Summary

Remove the unused backend field from scheduled-task execution templates so the protocol and write path stop carrying stale session backend state.

This keeps old stored records readable while preventing new scheduled tasks from persisting a field that nothing consumes.

Fixes #3306

Verification

• npm install
• npm --workspace @maka/core test
• npm --workspace @maka/runtime-host test (full suite started; the targeted scheduled-task protocol test also passed directly)
• npm run typecheck --workspace @maka/storage
• npm run typecheck --workspace @maka/runtime
• npm run typecheck --workspace @maka/runtime-host
• npx biome format packages/core/src/scheduled-task.ts packages/runtime-host/src/protocol/scheduled-task.ts packages/runtime-host/src/server/scheduled-task-coordinator.ts packages/runtime-host/src/__tests__/scheduled-task-protocol.test.ts
• npx biome lint packages/core/src/scheduled-task.ts packages/runtime-host/src/protocol/scheduled-task.ts packages/runtime-host/src/server/scheduled-task-coordinator.ts packages/runtime-host/src/__tests__/scheduled-task-protocol.test.ts
• node --test dist/__tests__/scheduled-task-protocol.test.js in packages/runtime-host

AI use

• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex contributed to issue discovery, repository analysis, implementation, validation, and PR preparation.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — old stored scheduled-task records remain readable, while new writes stop persisting an unused field