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/tools/filesystem.py

@@ -65,11 +65,12 @@ class ReadFileTool(BaseTool):
                 error=str(exc),
             )
         except FileNotFoundError as exc:
+            filename = Path(file_path).name
             return ToolResult(
                 tool_call_id=tool_call_id,
                 tool_name=self.name,
                 status=ToolResultStatus.ERROR,
-                error=str(exc),
+                error=f"{exc}. Use find_files to locate it, e.g. find_files(pattern=\"{filename}\")",
             )
         except FileSizeError as exc:
             return ToolResult(