fix: consume ACP agent greeting on cold-start before first real prompt

When an acpx session is created or reconnected, the ACP agent (e.g.
Claude Code) emits a cold-start greeting ("The model has been set
to …") as its first response. This greeting is returned as the result
of the first real pipeline prompt, causing stage outputs like goal.md
to contain the greeting text instead of actual research content.

Add a disposable warm-up prompt ("Respond with OK") in _ensure_session()
immediately after session creation. This consumes the greeting so that
subsequent chat() calls receive genuine responses.

Reproducer:
  researchclaw run --topic "..." --mode co-pilot
  → Stage 1 goal.md contains "The model has been set to … How can I
    help you?" instead of a SMART research goal.

Author: VahidAlizadeh

researchclaw/llm/acp_client.py

@@ -197,7 +197,13 @@ def _abs_cwd(self) -> str:
         return os.path.abspath(self.config.cwd)
 
     def _ensure_session(self) -> None:
-        """Find or create the named acpx session."""
+        """Find or create the named acpx session.
+
+        After creating or reconnecting a session, sends a disposable warm-up
+        prompt.  Without this, the agent's cold-start greeting (e.g. "The
+        model has been set to …") is returned as the response to the first
+        real prompt, swallowing the actual request.
+        """
         if self._session_ready:
             return
         acpx = self._resolve_acpx()
@@ -225,6 +231,20 @@ def _ensure_session(self) -> None:
                 raise RuntimeError(
                     f"Failed to create ACP session: {result.stderr.strip()}"
                 )
+
+        # Warm-up: consume the agent's cold-start greeting so it does not
+        # pollute the first real prompt's response.
+        try:
+            subprocess.run(
+                [acpx, "--approve-all", "--ttl", "0", "--cwd", self._abs_cwd(),
+                 self.config.agent, "-s", self.config.session_name,
+                 "Respond with OK"],
+                capture_output=True, text=True, encoding="utf-8",
+                errors="replace", timeout=60,
+            )
+        except Exception:  # noqa: BLE001
+            logger.debug("ACP warm-up prompt failed (non-fatal)")
+
         self._session_ready = True
         logger.info("ACP session '%s' ready (%s)", self.config.session_name, self.config.agent)