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'

.env.example

@@ -8,7 +8,7 @@ AZURE_TTS_VOICE=en-US-Ava:DragonHDOmniLatestNeural
 # LLM (OpenWebUI on xNAS, OpenAI-compatible; also serves the local llama.cpp
 # models. The model can be switched live from the web UI settings sheet.)
 LLM_BASE_URL=http://192.168.86.2:3005/api
-LLM_MODEL=gemma-4-e4b
+LLM_MODEL=qwen2.5-vl-3b
 LLM_API_KEY=sk-your-openwebui-api-key
 
 # LiveKit (generated in livekit.yaml, referenced here for the agent)