Modernize the challenge system

[minoneer]

Summary

The current challenge system mixes several concerns in one place:

• challenge type selection
• requirement evaluation
• completion and repeat handling
• cooldown/reset behavior
• freeform config semantics

That makes the system harder to extend and harder to reason about when adding new challenge behavior.

Goal

Modernize the challenge system around clearer extension points and simpler completion logic.

Candidate work

• add a custom requirements API
• support permission-based requirements
• reduce or remove hard-coded challenge type branching
• clean up completion, repeat, and cooldown logic

Related issues

• feature request: permission challenges #28 permission-based challenges
• Localize default challenge text from challenges.yml #114 localize default challenge text from challenges.yml

Notes

This issue is intended as an umbrella for challenge-system modernization. It should be broken into smaller implementation issues when the work starts.