Brings the Python SDK toward parity with the @cueapi/mcp 0.3.0 + 0.4.0
surface. Adds four new methods covering the SEND side (fire) and the
RECEIVE side (claimable list, claim, claim-next). Heartbeat signature
fix is held for a follow-up commit pending technical review.

CuesResource:
- fire(cue_id, payload_override=None, merge_strategy=None)
  POST /v1/cues/{id}/fire. For ad-hoc one-shot triggers and for using
  cues as a messaging channel between agents (carry message, instruction,
  task, reply_cue_id in payload_override).

ExecutionsResource:
- list_claimable(task=None, agent=None)
  GET /v1/executions/claimable?task=&agent=
  Filters server-side via query params (NOT client-side). Required for
  single-purpose workers; client-side filter after fetch hits the LIMIT
  50 starvation bug fixed in the 2026-04-25 prod incident.

- claim(execution_id, worker_id=...)
  POST /v1/executions/{id}/claim
  Atomic; returns 409 if already claimed.

- claim_next(worker_id=..., task=None)
  POST /v1/executions/claim (no task) OR list+claim chain (with task).
  Server's claim endpoint does not accept a task filter today; with task
  the SDK fans out (list_claimable filtered, pick oldest, claim by ID).
  Tiny race window between list and claim is bounded by the atomic claim
  returning 409, in which case the caller retries.

Version: 0.1.3 -> 0.2.0. Aligned cueapi/__init__.py (had drifted to
0.1.2) with pyproject.toml at the same time.

Tests: 42 unit tests pass (was 30; +12 net). Mirrors the existing
MagicMock pattern in tests/test_executions_resource.py.

Pending follow-up:
- ExecutionsResource.heartbeat(execution_id) currently sends an empty
  body and does not include worker_id via the X-Worker-Id request
  header that the server reads from. Worker-id is what the server uses
  to enforce ownership on heartbeat (returns 403 on mismatch); without
  it the race-protection check is silently bypassed. A signature change
  to add worker_id is held pending technical review of the deprecation
  cadence (additive kwarg-only with default-warn-on-omit vs hard
  signature change in a major bump).

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>