Add Cursor Cloud specific instructions to AGENTS.md

[Balaxxe]

Summary

Adds a ## Cursor Cloud specific instructions section to AGENTS.md with non-obvious caveats discovered during cloud environment setup:

• No Docker/database/external services needed (pure CLI tool)
• Pre-existing pyright errors in local_converter.py:800 (upstream, not to be fixed)
• make check / scripts/test-safe.sh gracefully handles missing env/ virtualenv
• Non-interactive app invocation command
• All 730 tests mock Mistral API calls — no API key needed

Validation

• python3 -m flake8 . — 0 errors
• python3 -m pyright — 2 pre-existing errors (upstream)
• python3 -m pytest tests/ — 730 passed
• python3 main.py --test — self-test OK
• python3 main.py --mode markitdown --no-interactive — converted HTML to Markdown successfully

Risk

Low — only adds documentation, no code changes.