fix: validate command module contracts during load

[spierenburg]

What this PR brings

• Validates command modules during startup before adding them to the command registry.
• Skips modules whose BINDS or CHANS values are not lists.
• Skips modules missing a callable process function.
• Provides a fallback help description if a module does not define HELP.

Why it matters

MatterBot dynamically loads command modules. Without basic contract validation, one malformed module can fail later during command dispatch or help rendering, making the failure harder to connect to the bad module.

This PR fails bad modules early, logs a clear reason, and keeps the rest of the bot usable.

Validation

python3 -m py_compile matterbot.py