feat(core): add /challenge reviews command for listing challenges awaiting review

[MikkelHebel]

Summary

Adds a /challenge reviews [author] command to list challenges in a configurable "awaiting review" project status, optionally filtered by GitHub assignee

• GithubHandler.get_issues_by_status(): queries the Projects v2 board for items matching a given Status value + optional assignee
• New config: REVIEW_STATUS (default In Review) and REVIEW_LIMIT (default 10), both configurable via env var or CLI flag, same pattern as STATUS/CATEGORIES
• Message includes a link to the filtered GitHub project view, and per challenge, links to both the GitHub issue and its linked Discord channel (if any)
• Docs updated in README and .env.example

This closes #3

[github-actions]

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[MikkelHebel]

I have read the CLA Document and I hereby sign the CLA

[The0mikkel]

I'm setting this on hold, until #16 have been merged, as this PR needs to be updated aftwards.