Enable closing pr during resetting and re-downloading

[desmondwong1215]

All open pr by the user will be closed during resetting and re-downloading.

[VikramGoyal23]

LGTM!

[VikramGoyal23]

This only fetches the first 30 PRs, and while technically the better option would be to add a flag of the form --limit 9999999 for edge cases, it is extremely unlikely the user will ever open >30 PRs on their repo (unless done deliberately). So it may be fine to leave it this way.

[Copilot]

Pull request overview

Adds support for automatically closing a user’s open pull requests during exercise reset and re-download workflows, using GitHub CLI, and persists PR-related metadata in the exercise config to drive that behavior.

Changes:

• Introduce close_pr() helper in app/utils/github_cli.py to close all open PRs authored by the current user in a given repo.
• Extend ExerciseRepoConfig to include pr_number and pr_repo_full_name, and read them from .gitmastery-exercise.json.
• Invoke PR-closing during progress reset and when re-downloading over an existing exercise directory.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
app/utils/github_cli.py	Adds GH CLI helper to list and close open PRs authored by the user.
app/configs/exercise_config.py	Adds PR metadata fields to exercise repo config and reads them from JSON.
app/commands/progress/reset.py	Closes open PRs (when configured) before deleting the fork during reset.
app/commands/download.py	Attempts to close PRs from a prior exercise config when re-downloading over an existing directory.

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