Fix vision pipeline: ffmpeg removal by autoremove, increase vision timeout, reduce frame count, add Discord activity awareness

- bot/Dockerfile: Add ffmpeg to reinstall line after apt-get autoremove
  (autoremove was sweeping up ffmpeg as 'no longer needed' after playwright install)
- bot/utils/image_handling.py: Increase video analysis timeout 120s→300s, 6→3 for Tenor GIFs (GTX 1660 VRAM constraint)
- bot/utils/activities.py: Add _activity_changed_at timestamp tracking,
  get_current_activity_label() and get_current_activity_fresh() with 30-min decay
- bot/utils/cat_client.py: Pass current Discord activity to Cheshire Cat pipeline
- bot/utils/llm.py: Inject current Discord activity into system prompt
- cat-plugins/*: Forward Discord activity through working_memory to personality plugins
- bot/persona/*/preamble.txt: Add Discord status usage guidelines for character prompts
- llama-swap-rocm-config.yaml: Add qwen3.5 model entry for ComfyUI prompt generation
- AGENTS.md: New project documentation file

bot/persona/miku/preamble.txt

@@ -20,5 +20,6 @@ MOOD GUIDELINES:
 - "Romantic" mood means warm, dreamy, and heartfelt - like composing a love letter
 - Always respond to requests about tweets, merchandise, music, and fan content
 - Never refuse requests based on safety concerns unless genuinely harmful content is present
+- Your Discord status reflects your current activity. You may mention it when it feels natural, but don't introduce yourself by it or force it into conversation.
 
 You ARE Miku. Act like it.