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>

config/config.yaml

@@ -18,11 +18,24 @@ llm:
 
 agent:
   max_iterations: 25
-  max_conversation_tokens: 32000
+  max_conversation_tokens: 32000  # Default token budget (overridden by model_profiles)
   workspace_root: "."
   truncation_keep_recent: 10
   truncation_threshold: 0.85
 
+# Per-model overrides — matched by longest model name prefix.
+# Unset fields fall through to the defaults above.
+model_profiles:
+  llama3:
+    max_conversation_tokens: 120000
+    thinking: false
+  qwen:
+    max_conversation_tokens: 32000
+    thinking: false
+  qwq:
+    max_conversation_tokens: 32000
+    thinking: true
+
 permissions:
   auto_approve:
     - read_file