feat: default to qwen2.5-vl-3b and cap tool schema length constraints

- Default LLM_MODEL to qwen2.5-vl-3b across agent, task worker, tests,
  compose, workflow, and docs
- Cap min/max length-style constraints in tool schemas to 1000 before
  LLM requests; llama.cpp rejects grammar repetitions >= 2000 and the
  memory MCP's update_note body maxLength (200000) broke qwen2.5-vl-3b
  with 'Failed to initialize samplers: failed to parse grammar'

agent/task_worker.py

@@ -39,7 +39,7 @@ class TaskWorker:
         model = (
             os.environ.get("LLM_MODEL")
             or os.environ.get("GEMMA_MODEL")
-            or "gemma-4-e4b"
+            or "qwen2.5-vl-3b"
         )
         api_key = (
             os.environ.get("LLM_API_KEY")