fix: empty response handling, /no_think model gating, per-model profiles

- Detect empty LLM responses (no content, no tool calls) instead of
  silently treating them as task completion. Retries once without tools
  before warning the user.
- Gate /no_think system message and chat_template_kwargs to Qwen/QwQ
  models only — sending /no_think to llama3.x caused empty responses.
- Add model_profiles config section for per-model overrides (token
  budget, thinking, temperature, max_tokens) matched by name prefix.
  Applied at startup and on /model switch.
- Update SessionManager on /model switch so session files record the
  correct model.
- Add NDJSON fallback in SSE stream parser for Ollama compatibility.
- Improve read_file error to suggest find_files on FileNotFoundError.
- Add diagnostic logging for empty streams and empty results.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/services/session.py

@@ -52,6 +52,10 @@ class SessionManager:
         self._session_dir = workspace_root / config.session_dir
         self._session_id = f"{self._workspace_hash}_{datetime.now(UTC).strftime('%Y%m%d_%H%M%S')}"
 
+    def update_model(self, model: str) -> None:
+        """Update the model name for session metadata."""
+        self._model = model
+
     def save(self, ctx: "SessionContext") -> Path:
         """Save session state to a JSON file via atomic write.