feat(#7): add subtask support to task management API

[fullsend-ai-coder]

Summary

• Add subtask support to the task management API, enabling users to break tasks into smaller, trackable child tasks
• New endpoints: POST /tasks/:id/subtasks (create) and GET /tasks/:id/subtasks (list)
• Subtasks share core task properties (title, description, completed) and are linked via parentId
• Cascade delete removes all subtasks when a parent task is deleted
• Single-level nesting enforced — subtasks of subtasks are rejected with a 400 error
• Backward compatible: GET /tasks continues to return all tasks including subtasks

Changes

• src/store.js: Extended TaskStore with createSubtask(), listSubtasks(), cascade delete in delete(), and parentId field on all tasks
• src/index.js: Added route handlers for POST /tasks/:id/subtasks and GET /tasks/:id/subtasks
• src/store.test.js: Added 9 new tests covering subtask creation, nesting rejection, parent validation, listing, cascade delete, backward compatibility, and parent isolation

Testing

All 16 tests pass (7 existing + 9 new):

1. Create subtask under valid parent → 201 with correct parentId
2. Create subtask under non-existent parent → error
3. Reject subtask of a subtask (single-level nesting)
4. List subtasks of a parent → returns only that parent's subtasks
5. List subtasks of non-existent parent → null
6. List subtasks when parent has none → empty array
7. Cascade delete parent → all subtasks also deleted
8. GET /tasks returns all tasks including subtasks (backward compat)
9. Only returns subtasks belonging to the specified parent

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Closes #7

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🤖 Finished Review · ✅ Success · Started 8:06 AM UTC · Completed 8:20 AM UTC

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