feat(gateway): expose subagent tool calls and thinking to user (fixes #169) (#186)

When subagents run via delegate_task, the user now sees real-time
progress instead of silence:

CLI: tree-view activity lines print above the delegation spinner
  🔀 Delegating: research quantum computing
     ├─ 💭 "I'll search for papers first..."
     ├─ 🔍 web_search  "quantum computing"
     ├─ 📖 read_file  "paper.pdf"
     └─ ⠹ working... (18.2s)

Gateway (Telegram/Discord): batched progress summaries sent every
5 tool calls to avoid message spam. Remaining tools flushed on
subagent completion.

Changes:
- agent/display.py: add KawaiiSpinner.print_above() to print
  status lines above an active spinner without disrupting animation.
  Uses captured stdout (self._out) so it works inside the child's
  redirect_stdout(devnull).

- tools/delegate_tool.py: add _build_child_progress_callback()
  that creates a per-child callback relaying tool calls and
  thinking events to the parent's spinner (CLI) or progress
  queue (gateway). Each child gets its own callback instance,
  so parallel subagents don't share state. Includes _flush()
  for gateway batch completion.

- run_agent.py: fire tool_progress_callback with '_thinking'
  event when the model produces text content. Guarded by
  _delegate_depth > 0 so only subagents fire this (prevents
  gateway spam from main agent). REASONING_SCRATCHPAD/think/
  reasoning XML tags are stripped before display.

Tests: 21 new tests covering print_above, callback builder,
thinking relay, SCRATCHPAD filtering, batching, flush, thread
isolation, delegate_depth guard, and prefix handling.

agent/display.py

@@ -199,6 +199,21 @@ class KawaiiSpinner:
     def update_text(self, new_message: str):
         self.message = new_message
 
+    def print_above(self, text: str):
+        """Print a line above the spinner without disrupting animation.
+
+        Clears the current spinner line, prints the text, and lets the
+        next animation tick redraw the spinner on the line below.
+        Thread-safe: uses the captured stdout reference (self._out).
+        Works inside redirect_stdout(devnull) because _write bypasses
+        sys.stdout and writes to the stdout captured at spinner creation.
+        """
+        if not self.running:
+            self._write(f"  {text}", flush=True)
+            return
+        # Clear spinner line, print text above, spinner redraws on next tick
+        self._write(f"\r\033[K  {text}", flush=True)
+
     def stop(self, final_message: str = None):
         self.running = False
         if self.thread: