feat(python-sdk): expose subagent task query API

[ZhiXiao-Lin]

Stacked on #38 → #37 → #36 → #35. Base retargets to `main` automatically as upstream PRs land.

Summary

Mirrors the new `AgentSession` subagent query API (introduced in #35) onto the Python SDK so callers can introspect delegated subagent tasks symmetrically with the Node SDK (#37).

```python
snap = session.subagent_task('task-abc')

dict with keys: task_id, parent_session_id, child_session_id, agent,

description, status, started_ms, updated_ms, finished_ms?, output?,

success?, progress: [...] (status ∈ {'running','completed','failed'})

or None when the task id is unknown.

all_tasks = session.subagent_tasks() # list (this session, oldest first)
pending = session.pending_subagent_tasks() # list (status == 'running' only)
```

Each binding follows the existing run-query pattern: drops the GIL via `py.allow_threads`, blocks on the shared tokio runtime, then serializes the result through serde_json so Python sees plain `dict` / `list` / `None`.

Test plan

• `cargo check --lib` on the Python SDK crate is clean.
• `maturin build --release` produces a wheel.
• New `tests/test_subagent_query_api.py` runs the wheel in a venv and asserts:
  • `subagent_tasks()` → `[]` for a fresh session
  • `pending_subagent_tasks()` → `[]` for a fresh session
  • `subagent_task('task-does-not-exist')` → `None`
• Smoke uses an inline ACL string, so no real provider credentials are needed.

Follow-ups

• PR Orchestrator API: sub-agent event streaming and fine-grained permissive control #5: cancellation by task id — both SDKs gain `cancel_subagent_task(id)` once the core grows a `JoinHandle` + `CancellationToken` registry per subagent.