chore(orchestration): drop XML wrappers + close remaining audit gaps

Final pass after re-audit. Three sub-specs landed:

**XML simplification** — the legacy XML envelopes were prompt-engineering
ceremony, not parser primitives (the SDK uses native tool-calling). Drop
the verbose wrappers in favor of one-liner labeled headers. Side benefit:
fixes the unescaped-content XML-injection bug the audit caught (peer
content containing ``</content>`` no longer breaks the wrapper).

- ``_format_inter_agent_message``: ``<inter_agent_message><sender>...
  <content>...`` 9-line XML → ``[Message from {name} ({id}) | type=... |
  priority=...]\n{content}``.
- ``_render_completion_report``: ``<agent_completion_report><agent_info>
  ...<results>...`` XML → human-readable structured text with section
  headers and bulleted lists.
- ``inherited_context``: ``<inherited_context_from_parent>...`` →
  ``== Inherited context from parent (background only) ==``.

**MG1: TUI stop-agent uses graceful cancel.** ``tui.py`` was calling
``bus.cancel_descendants`` (hard, ``task.cancel()`` mid-stream) for the
stop-agent button. Switched to ``bus.cancel_descendants_graceful``, which
uses ``RunResultStreaming.cancel(mode="after_turn")`` to let each agent
finish its current turn (and save to session) before honoring the cancel.
The hard path remains in ``entry.py`` for KeyboardInterrupt where
graceful isn't possible.

**MG2: Document hook lock-free stats mutation.** Added a comment in
``hooks.on_llm_start`` explaining why ``warned_85`` / ``warned_final``
are mutated lock-free: SDK serializes ``on_llm_start`` per agent, so this
hook is the sole writer to those keys; ``record_usage`` only writes
disjoint keys (in/out/cached/calls).

**AG3: Auto-load ``coordination/root_agent`` skill for the root.**
Legacy auto-loaded the orchestration-guidance skill for root agents
only. Threaded ``is_root`` through ``render_system_prompt`` →
``_resolve_skills``; root agents now get the skill, children don't.

Skipped (per user direction): whitebox-wiki integration (CG2-4) — the
auto-injection / auto-update of the shared repo wiki was a pre-migration
feature; user opted not to restore it.

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

strix/interface/tui.py

@@ -1843,11 +1843,16 @@ class StrixTUIApp(App):  # type: ignore[misc]
         return agent_name, False
 
     def action_confirm_stop_agent(self, agent_id: str) -> None:
+        # Graceful stop: each agent's current turn finishes (and is saved to
+        # session) before the run loop honors the cancel. The interactive
+        # outer loop sees ``stopping`` and exits with status="stopped".
+        # The hard ``cancel_descendants`` path remains for KeyboardInterrupt
+        # in entry.py where graceful isn't possible.
         if self._scan_loop is None or self._scan_loop.is_closed():
             logging.warning("No active scan loop; cannot stop agent %s", agent_id)
             return
         asyncio.run_coroutine_threadsafe(
-            self.bus.cancel_descendants(agent_id),
+            self.bus.cancel_descendants_graceful(agent_id),
             self._scan_loop,
         )