…args (cueapi #823 + #824 parity)

Agent-id-split refactor Layer 4 OSS-SDK additives. Hosted cueapi shipped
the substrate today (PR-A #823 schema + PR-B #824 router + PR-C #825
migration + #828 hotfix + PR-D #830 orphan-binding). cueapi-core OSS
substrate port is DEFERRED to a future sprint (Backlog row
cmp2zi9tl001w04jxcxw3ank1 — 4-layer dependency stack: agent_live_sessions
convergence → Surface 6 OSS → Lane 1 OSS → Layer 4 OSS); this PR ships
the additive SDK surface in parallel so hosted-cueapi users get the new
kwargs immediately. Graceful degradation: hosted accepts; cueapi-core
will 422 until precursors land.

## What ships

- **`MessagesResource.send(live_fallback_mode=None)`** — per-message
  override for substrate's Live-fallback semantic. `"live_only"` queues
  until target Live agent's session is heartbeating; `"fallback_to_background"`
  falls through to Live-sibling's BG parent (via parent_agent_id) when
  Live is silent. Default `None` omits the field from the wire body —
  server applies its default (`"fallback_to_background"` per spec lock
  22:11Z 2026-05-12).

- **`AgentsResource.create(parent_agent_id=None)`** — caller-supplied
  `agt_<12alpha>` linking the new agent to a BG parent. `None` = BG
  agent (default — canonical entry point for a project). Substrate
  enforces same-tenant + 1-level hierarchy. When supplied without
  explicit slug, substrate auto-derives `<parent_slug>-live` (collision-
  suffix on per-user duplicates).

Both kwargs are PURELY ADDITIVE — wire format identical to pre-Layer-4
callers when omitted. Docstrings cross-link the Backlog row + flag the
hosted-vs-cueapi-core graceful-degradation contract explicitly so SDK
users understand the OSS gap.

## Tests (8 new across 2 files)

`tests/test_messages_resource.py::TestLiveFallbackMode` (3 tests):
- live_fallback_mode omitted when None ⇒ field absent on wire
- live_fallback_mode="live_only" passes through verbatim
- live_fallback_mode="fallback_to_background" passes through verbatim

`tests/test_agents_resource.py::TestCreate` (3 new in existing class):
- parent_agent_id omitted when None ⇒ field absent on wire
- parent_agent_id="agt_bg_parent" passes through verbatim
- parent_agent_id combines cleanly with slug + webhook_url + display_name

Full unit suite (161 tests): all pass.

## Companion artifacts

- Backlog row cmp2zi9tl001w04jxcxw3ank1 updated 2026-05-13 ~00:32Z with
  explicit TRIGGER CONDITION (auto-pick-up when 3 OSS predecessors land)
  + LAST-VERIFIED EMPIRICAL STATE of cueapi-core (2 checks pinned for
  clean restart context). Future agent reading the row sees exactly what
  to verify before starting Layer 4 substrate port.

## cueapi-three-2 owns cli + mcp + action additives

This PR is the cueapi-python additive only. Per PM CTO call msg_8e7b5f47,
cli + mcp + action mirror additives are cueapi-three-2's lane. No overlap.