refactor: nuke gratuitous XML serialization + delete argument_parser

Argument parser:
- Delete ``strix/tools/argument_parser.py`` and its tests. The SDK
  validates and types tool arguments via Pydantic before they hit our
  wrappers, and the in-container tool server receives JSON-typed
  kwargs over the wire. The string-coercion belt-and-suspenders is no
  longer pulling its weight.

XML → JSON / typed structures:
- ``create_vulnerability_report``: ``cvss_breakdown`` is now a
  ``dict[str, str]`` of the 8 metrics; ``code_locations`` is a
  ``list[dict]``. No more XML parsing in the tool or the renderer.
- ``check_duplicate``: the dedup judge now emits a single JSON object
  instead of an ``<dedupe_result>`` block. Strict JSON parser handles
  optional code-fence wrappers.
- ``agent_finish``: completion report posted to the parent inbox is a
  JSON object (``kind``, ``from``, ``agent_id``, ``success``,
  ``summary``, ``findings``, ``recommendations``) rather than a
  hand-rolled ``<agent_completion_report>`` XML envelope.
- ``create_agent``: identity preamble + inherited-context markers are
  plain bracketed labels rather than ``<agent_delegation>`` /
  ``<inherited_context_from_parent>`` envelopes.
- ``inject_messages_filter``: peer messages get a
  ``[Message from agent <id> | type=... | priority=...]`` header line
  instead of an ``<inter_agent_message>`` envelope.
- Crash + system-warning messages: bracketed labels, no XML.
- System prompt: the inter-agent block now describes the new header
  format and drops the "never echo XML envelope" rule.
- ``strix/llm/utils.py``: deleted. ``clean_content`` collapsed into a
  one-line blank-line normalizer in the agent-message renderer (the
  XML envelope scrub had nothing left to scrub).

Tests updated to match the new shapes.

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

tests/orchestration/test_filter.py

@@ -53,14 +53,14 @@ async def test_pending_messages_appended_in_order() -> None:
 
     assert len(out.input) == 3
     assert out.input[0] == {"role": "user", "content": "task"}
-    assert "<inter_agent_message from='b'" in out.input[1]["content"]
+    assert "Message from agent b" in out.input[1]["content"]
     assert "hello" in out.input[1]["content"]
     assert "second" in out.input[2]["content"]
-    assert "priority='high'" in out.input[2]["content"]
+    assert "priority=high" in out.input[2]["content"]
 
 
 @pytest.mark.asyncio
-async def test_user_sender_skips_xml_wrap() -> None:
+async def test_user_sender_skips_envelope() -> None:
     bus = AgentMessageBus()
     await bus.register("a1", "alpha", parent_id=None)
     await bus.send("a1", {"from": "user", "content": "follow-up question"})

tests/orchestration/test_hooks.py

@@ -93,7 +93,7 @@ async def test_on_llm_end_records_usage_and_increments_turn() -> None:
 
 @pytest.mark.asyncio
 async def test_on_agent_end_detects_crash() -> None:
-    """C8 (AUDIT_R2): on_agent_end without agent_finish_called posts crash to parent."""
+    """on_agent_end without agent_finish_called posts crash message to parent."""
     hooks = StrixOrchestrationHooks()
     bus = AgentMessageBus()
     await bus.register("root", "root", parent_id=None)
@@ -104,8 +104,9 @@ async def test_on_agent_end_detects_crash() -> None:
 
     drained = await bus.drain("root")
     assert len(drained) == 1
-    assert "<agent_crash" in drained[0]["content"]
-    assert "agent_id='child'" in drained[0]["content"]
+    assert "[Agent crash]" in drained[0]["content"]
+    assert "child" in drained[0]["content"]
+    assert drained[0]["type"] == "crash"
     assert bus.statuses["child"] == "crashed"