Add development environment setup (AGENTS.md)

[as567-code]

Summary

Adds AGENTS.md with Cursor Cloud-specific development environment instructions for the Video Understanding & Chat System.

What's included

• AGENTS.md: Documents how to run the Streamlit app in a cloud/Linux environment, including:
  • Required HF_HOME environment variable override (the repo hardcodes a macOS SSD path)
  • App startup command with headless mode flags
  • Notes on ML model downloads, Gemini API key requirement, and absence of linter/test suite

Environment setup verified

• Python 3.12 virtual environment created with all dependencies from requirements.txt
• Streamlit app starts and serves on port 8501
• Full end-to-end workflow tested: video upload → frame extraction → BLIP captioning → Flan-T5 chat Q&A