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/streaming.py

@@ -60,8 +60,10 @@ class StreamHandler:
         """
         thinking_notified = False
         last_update_time = 0.0
+        chunk_count = 0
 
         async for chunk in chunk_iter:
+            chunk_count += 1
             self._process_chunk(chunk)
 
             if not self._display_config.stream_output:
@@ -96,6 +98,14 @@ class StreamHandler:
             self._on_done()
 
         tool_calls = self._build_tool_calls() or None
+
+        if chunk_count > 0 and not self._accumulated_content and not tool_calls:
+            logger.debug(
+                "stream_empty_result",
+                chunks_received=chunk_count,
+                had_reasoning=bool(self._accumulated_reasoning),
+            )
+
         return Message(
             role="assistant",
             content=self._accumulated_content or None,