feat(desktop): stream subagent activity into watch windows (#47060)

* feat(desktop): stream subagent replies into watch windows

A desktop watch window resumes a child session lazily (no full agent) and
mirrors the parent-relayed `subagent.*` events into native child-session
stream events. The child's streamed reply text was never relayed, so the
window sat blank while the subagent "talked".

- delegate_tool: forward the child's `run_conversation` stream tokens up the
  progress relay as `subagent.text` (inert under CLI/TUI — their progress
  handlers ignore non-tool event types; only a gateway watch window mirrors it).
- server: mirror `subagent.text` -> `message.delta` on the child sid only, and
  skip the parent emit (per-token frames are meaningless on the parent session,
  which shows the child via the spawn tree). Demote `subagent.start` to a
  one-time goal header and drop the noisy `subagent.progress` mirror — tools
  already mirror natively.
- server: guard `_start_agent_build` so a lazy watch session spectating an
  in-flight child stays lazy; incidental RPCs were upgrading it to a full
  agent mid-stream and silently killing the mirror.

* fix(desktop): keep watch-window chat clear of titlebar chrome

Secondary windows (new-session scratch, subagent watch, cmd-click pop-out)
hide the titlebar tool cluster + session header, so the transcript ran to the
window's top edge and streamed text slid up under the OS traffic lights.

- Gate the hidden chrome on `isSecondaryWindow()` everywhere (app-shell,
  chat header, thread list) instead of the narrower new-session flag.
- Add a fixed opaque drag-strip at the top of the secondary-window transcript:
  content padding alone scrolls away with the text, so the strip masks
  anything behind it and keeps the window draggable like the main header.

* fix: WSL subagent window

* fix: subagent window top padding

---------

Co-authored-by: Austin Pickett <pickett.austin@gmail.com>
Co-authored-by: Teknium <127238744+teknium1@users.noreply.github.com>

tools/delegate_tool.py

@@ -867,6 +867,15 @@ def _build_child_progress_callback(
             _relay("subagent.complete", preview=preview, **kwargs)
             return
 
+        if event_type == "subagent.text":
+            # Streamed assistant reply text from the child. Relay verbatim so a
+            # gateway watch window can mirror the child "talking" as it streams.
+            # No spinner echo — the CLI shows the child via the tree, and the
+            # CLI/TUI progress handlers ignore non-tool event types, so this is
+            # inert there; only a gateway watch window consumes it.
+            _relay("subagent.text", preview=preview)
+            return
+
         # Normalise legacy strings, new-style "delegate.*" strings, and
         # DelegateEvent enum values all to a single DelegateEvent.  The
         # original implementation only accepted the five legacy strings;
@@ -1626,11 +1635,23 @@ def _run_single_child(
         # Python stack (see #14726 — 0-API-call hangs are opaque without it).
         _worker_thread_holder: Dict[str, Optional[threading.Thread]] = {"t": None}
 
+        def _relay_child_text(delta: str) -> None:
+            # Forward the child's streamed reply text up the progress relay so
+            # gateway watch windows mirror it live (subagent.text → message.delta).
+            # Inert under CLI/TUI: their progress handlers ignore non-tool events.
+            if not delta or not child_progress_cb:
+                return
+            try:
+                child_progress_cb("subagent.text", preview=delta)
+            except Exception as e:
+                logger.debug("Child text relay failed: %s", e)
+
         def _run_with_thread_capture():
             _worker_thread_holder["t"] = threading.current_thread()
             return child.run_conversation(
                 user_message=goal,
                 task_id=child_task_id,
+                stream_callback=_relay_child_text,
             )
 
         _child_future = _timeout_executor.submit(_run_with_thread_capture)