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.

strix/agents/factory.py

@@ -151,6 +151,7 @@ def build_strix_agent(
         skills=skills,
         scan_mode=scan_mode,
         is_whitebox=is_whitebox,
+        is_root=is_root,
         interactive=interactive,
         system_prompt_context=system_prompt_context,
     )

strix/agents/prompt.py

@@ -27,6 +27,7 @@ def _resolve_skills(
     requested: list[str] | None,
     scan_mode: str = "deep",
     is_whitebox: bool = False,
+    is_root: bool = False,
 ) -> list[str]:
     """Build the deduped, ordered skills list for the prompt render.
 
@@ -36,11 +37,15 @@ def _resolve_skills(
     2. ``scan_modes/<mode>`` (always).
     3. ``tooling/agent_browser`` (always — every agent has shell + the
        agent-browser CLI).
-    4. Whitebox-specific skills if applicable.
+    4. ``coordination/root_agent`` for the root agent only — orchestration
+       guidance for delegating to specialist subagents.
+    5. Whitebox-specific skills if applicable.
     """
     ordered: list[str] = list(requested or [])
     ordered.append(f"scan_modes/{scan_mode}")
     ordered.append("tooling/agent_browser")
+    if is_root:
+        ordered.append("coordination/root_agent")
     if is_whitebox:
         ordered.append("coordination/source_aware_whitebox")
         ordered.append("custom/source_aware_sast")
@@ -59,6 +64,7 @@ def render_system_prompt(
     skills: list[str] | None = None,
     scan_mode: str = "deep",
     is_whitebox: bool = False,
+    is_root: bool = False,
     interactive: bool = False,
     system_prompt_context: dict[str, Any] | None = None,
 ) -> str:
@@ -70,6 +76,8 @@ def render_system_prompt(
             skill.
         is_whitebox: When True, the source-aware whitebox skill stack
             is loaded too.
+        is_root: When True, ``coordination/root_agent`` orchestration
+            guidance is auto-loaded.
         interactive: When True, the prompt renders the interactive-mode
             communication rules block.
         system_prompt_context: Free-form dict that the template's
@@ -96,6 +104,7 @@ def render_system_prompt(
             requested=skills,
             scan_mode=scan_mode,
             is_whitebox=is_whitebox,
+            is_root=is_root,
         )
         skill_content = load_skills(skills_to_load)
         env.globals["get_skill"] = lambda name: skill_content.get(name, "")