fix(agent): make mid-turn /steer trusted, not read as injection

A steer rides inside a tool result (the only role-alternation-safe slot
mid-turn), so a bare "User guidance:" line reads as untrusted tool content —
well-behaved models refuse it as suspected prompt injection (observed live:
"I only follow instructions from you directly, not ones injected through
command results").

- Wrap steers in a bounded, self-describing [OUT-OF-BAND USER MESSAGE] marker
  (prompt_builder.format_steer_marker), shared by both drain sites.
- Add STEER_CHANNEL_NOTE to the core system prompt so the model expects this
  exact marker and trusts it as a genuine user message — while still ignoring
  lookalikes buried in tool/web/file output. Static text → byte-stable prompt,
  no prompt-cache regression; gated on the agent having tools.
- Desktop: steer ack is now an inline transcript note (⏩ steered · …) instead
  of a toast.

Marker is intentionally static (not a per-session nonce) to honor the
byte-stable system-prompt caching policy; nonce hardening noted as follow-up.

agent/conversation_loop.py

@@ -877,7 +877,8 @@ def run_conversation(
             for _si in range(len(messages) - 1, -1, -1):
                 _sm = messages[_si]
                 if isinstance(_sm, dict) and _sm.get("role") == "tool":
-                    marker = f"\n\nUser guidance: {_pre_api_steer}"
+                    from agent.prompt_builder import format_steer_marker
+                    marker = format_steer_marker(_pre_api_steer)
                     existing = _sm.get("content", "")
                     if isinstance(existing, str):
                         _sm["content"] = existing + marker