feat(migration): phase 3 — multi-agent graph tools + Runner bridge

Six SDK function tools that drive the AgentMessageBus from Phase 0,
replacing the legacy _agent_graph / _agent_messages / _agent_instances
globals:

- view_agent_graph: render parent/child tree from bus.parent_of with a
  per-status summary (running / waiting / completed / crashed / stopped).
- agent_status: per-agent lifecycle + pending-message count snapshot.
- send_message_to_agent: queue into bus.inboxes; rejects sends to
  finalized targets so the model gets feedback rather than a silent
  drop (the bus's own send method drops to support the C13 cleanup,
  but the tool surfaces it as a structured error).
- wait_for_message: poll inbox once per second up to timeout. Polling
  rather than asyncio.Event because a missed wakeup on Event would be
  hard to debug; the bus already serializes through its own lock.
- create_agent: spawn a child via asyncio.create_task(Runner.run(...)).
  Pulls an agent_factory callable from ctx.context (the Phase 5 root
  assembly is the one that wires it in). Registers the child with the
  bus before the task starts, stores the task handle in bus.tasks so
  cancel_descendants can cascade (C9), builds the child's identity
  block + optional inherited parent context, and runs the child with
  StrixOrchestrationHooks.
- agent_finish: subagent-only termination. Flips agent_finish_called
  so the on_agent_end hook records "completed" instead of "crashed"
  (C8), and posts a structured <agent_completion_report> XML envelope
  to the parent's inbox.

run_config_factory.make_agent_context grows two fields: sandbox_client
(reused across child runs) and agent_factory (Phase 3 needs it; Phase 5
fills it in). PLC0415 fixed by hoisting the openai.types.shared.Reasoning
import to module-level.

Tests: 17 new tests in test_sdk_graph_tools.py — registration, all six
tools' happy and error paths, real AgentMessageBus integration so the
tools exercise production code paths, create_agent verified for spawn
shape (task created, bus registered, identity block in input) plus a
bus.cancel_descendants integration check.

Refs: PLAYBOOK.md §4.3, AUDIT_R2 §1.4 (cancel_descendants), AUDIT_R3 C8.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -298,6 +298,7 @@ ignore = [
 "strix/tools/terminal/terminal_sdk_tool.py" = ["TC002"]
 "strix/tools/python/python_sdk_tool.py" = ["TC002"]
 "strix/tools/proxy/proxy_sdk_tools.py" = ["TC002"]
+"strix/tools/agents_graph/agents_graph_sdk_tools.py" = ["TC002"]
 # Sandbox dispatch helper has many short-circuit error returns (auth fail,
 # size cap, decode fail, etc). Each is a distinct, documented failure mode
 # the model needs to see verbatim — collapsing them harms readability.