Add progress bar for Git clone and fetch operations. (#1858)

[GoDiao]

Summary

• Git clone and fetch now show a live progress bar instead of a static log message
• New `briefcase.git.GitProgress` class wraps `GitPython`'s `RemoteProgress` interface with Rich progress bars
• Progress displays: stage label (counting, receiving, resolving deltas), bar, percentage, and time remaining
• Idempotent cleanup via context manager pattern (try/finally)

Related issues

Closes #1858.

PR Principals

• Compatibility: No breaking changes
• Upgradeability: Pure additive change
• No bloat: Uses existing Rich dependency
• Elegance: ~70 LOC in a dedicated module with clean separation of concerns"

Demo

[freakboy3742]

Thanks for the PR.

There's a couple of problems with this PR that need to be resolved before we can review it.

First - you've deleted the PR template and replaced it with your own. I'm guessing this has been done by whatever AI tool you've used to generate the PR; however, it's a problem because our PR template includes some content that we need you to acknowledge - in particular, that you've acknowledged the use of any AI tooling, in accordance with our AI policy

Secondly - even without that, your PR has failed automated CI checks, because you haven't added a changenote. The PR template asks you to acknowledge that you've read our contribution guide - which points out the need for adding a changenote.