feat: add pr checkout command

[hex2dec]

Summary

• add gitee pr checkout to fetch and switch to an existing pull request branch
• report created vs existing branch state in text and JSON output
• cover missing PRs, invalid local git context, and checkout conflicts with fixture-based tests

Testing

• cargo test -q
• cargo fmt -- --check
• cargo clippy --all-targets --all-features -- -D warnings

Closes #9