Document the learner.py hot-reload caveat in README

[jeanclawd]

Problem

`CLAUDE.md` notes that editing `frontend/py/learner.py` requires a hard reload (Cmd-Shift-R / Ctrl-Shift-R) for Pyodide to re-import the module — a normal reload re-fetches the file but the module stays cached.

A first-time contributor who edits Python code, refreshes, and sees no change will assume their change failed and burn 10 minutes debugging.

Proposal

Add an explicit note to the README "Running locally" section (or a dedicated `CONTRIBUTING.md` / `DEVELOPMENT.md`):

⚠️ Editing `frontend/py/learner.py` requires a hard reload (Cmd-Shift-R on macOS, Ctrl-Shift-R on Linux/Windows) for Pyodide to re-import the module. A normal reload re-fetches the file but the import cache survives.

Same note can apply if other Python files are added later.

Acceptance criteria

• README mentions the hard-reload requirement near the local-development instructions.
• The note is discoverable from the top-level README (not buried deep).