Add requirements.txt (markdown2) so Cloudpepper installs the dependency on attach #1

pan_ai_pro/__manifest__.py declares external_dependencies: {'python': ['markdown2']}, but the repo has no requirements.txt.

Cloudpepper's auto_requirements only runs pip install -r requirements.txt, so attaching this repo to an instance where pan_ai_pro is already installed in the database fails at import:

File ".../pan_ai_pro/models/ai_agent.py", line 8, in <module>
    from markdown2 import markdown as md_convert
ModuleNotFoundError: No module named 'markdown2'

The registry then fails to load and the whole instance returns HTTP 500 until markdown2 is installed by hand in the instance venv. Odoo.sh hides this because it generates requirements from manifests.

Fix: add a root-level requirements.txt containing markdown2.

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