[codex] add task scheduling and management APIs

[DorianZheng]

Summary

• add public SDK task types plus scheduleTask, listTasks, getTask, updateTask, pauseTask, resumeTask, and cancelTask
• wire the task APIs through AgentImpl, centralize next-run/task-id helpers, and expose task run history from the database
• add task API, lifecycle, and SDK type coverage, plus an SDK E2E script and updated docs/README examples
• clean up agent deletion so custom agent workdirs are removed without risking the shared registry DB

Why

The scheduler and MCP layer already supported task execution, but the public Node SDK did not expose a complete task management surface. That made it hard for SDK consumers to schedule tasks, inspect runs, or manage task lifecycle programmatically.

Impact

SDK consumers can now create and manage scheduled tasks directly through the public API, including reading task details and run history. The docs and README now show the task flow, and the added tests cover persistence and end-to-end lifecycle behavior.

Validation

• npm run typecheck
• npm test -- --run src/task-api.test.ts src/task-lifecycle.test.ts src/db.test.ts src/sdk-types.test.ts src/agentlite-impl.test.ts
• npx eslint src/agent-impl.ts src/task-lifecycle.test.ts

Notes

• npm run lint still reports existing unrelated repo-wide lint errors outside this change set, so validation here focused on the touched task-path files and tests.