Bind the node emitter's session into its durable events

[Parad0x-Labs]

What failed

tests/test_runtime_events_carry_the_client_turn_id.py — 4 of 7 cases red in the main CI
census (run 35570948370) and in every PR run since; failing since the public root
(78f818b), reproducing locally 4/4 deterministic:

• 3 × AttributeError: 'object' object has no attribute '_emit_runtime_event' at
  core/agent_runtime/agent.py:3681 (raised inside worker threads)
• test_node_events_carry_the_client_turn_id_through_the_real_runner: 0 events where 4
  were asserted

Root cause (product defect, not just a stale test rig)

Two layers:

1. The emitter drops its own session. _agent_node_emitter(session_id, source_context)
   documents itself as the per-session durable emitter ("A per-turn emitter putting node
   lifecycle on the durable stream the UI polls") and accepts session_id as its first
   parameter — but the closure never uses it. The streaming seam it routes through
   (_emit_runtime_event → orchestrator.emit_runtime_event →
   core.runtime_task_events.emit_runtime_event) persists only what the source context
   names (runtime_session_id/session_id). A turn whose context carries no session key —
   e.g. the conductor handing the raw ask context — wrote its agent_node_* rows nowhere:
   the blank-Agents-panel regression class this emitter exists to prevent (its own docstring
   cites the 2026-08-29 IDLE-companion incident).
2. The test rig predates the streaming route. The suite's helper bound the factory with a
   bare object() ("It reads nothing off self" — true when the emitter wrote through a
   module-level append; false since the streaming seam reads self._emit_runtime_event).

Repair

• core/agent_runtime/agent.py: the emitter captures a sink_context once at creation
  (same concurrency law as the tags it already captures on the turn's thread) with
  setdefault("session_id", session_id) — additive, never overrides a session the caller's
  context already names; the per-emit call routes through the streaming seam with it.
• tests/test_runtime_events_carry_the_client_turn_id.py: the helper binds the factory to a
  carrier of exactly the two production mixins the streaming route reads
  (ToolResultWorkflowSurfaceMixin for _emit_runtime_event,
  RuntimeCheckpointSupportMixin for the checkpoint resolver) — both stateless adapters, so
  the real emission code still runs without booting an agent. No assertion changed.

Validation

• Target suite: 4 failed → 7 passed (includes the real-runner case asserting exactly 4
  node events, no duplicates, when the context already names a session — the setdefault
  no-op path).
• Cumulative neighbors — the twelve suites touching the node-emitter seam
  (test_a_running_node_is_visible_while_it_runs, test_agent_node_events_reach_companion,
  test_node_events_reach_the_pollable_stream, test_chat_companion_laws,
  test_the_agents_panel_shows_work_while_it_happens, test_worker_effect_context_p0, …):
  178 passed, 3 failed — byte-identical result on clean main (the 3 are pre-existing:
  2 × worker_effect served weather + 1 × event_log untagged grouping; stash-verified A/B).
• ruff check . clean (pinned 0.16.7).

Base: main 8153a96. Head: 3b19e6b. Files: core/agent_runtime/agent.py (+10),
tests/test_runtime_events_carry_the_client_turn_id.py (helper only, assertions unchanged).