fix(app-server): run git API commands without blocking Tokio handlers

[694410194]

Summary

• switch the app-server git API endpoints from std::process::Command to tokio::process::Command
• await every git subprocess inside the existing async handlers so git status/diff/log/branch/stash routes stop blocking Tokio worker threads
• keep the response parsing and error handling behavior unchanged while removing the synchronous process bottleneck

Fixes PlatformNetwork/bounty-challenge#53205
/claim #53205

Verification

• cargo fmt --manifest-path /tmp/cortex-bounty/Cargo.toml --package cortex-app-server
• cargo check --manifest-path /tmp/cortex-bounty/Cargo.toml -p cortex-app-server
• cargo test --manifest-path /tmp/cortex-bounty/Cargo.toml -p cortex-app-server --lib -- --skip tools::shell::tests::test_execute_shell
• git diff --check

Notes

• cargo test --manifest-path /tmp/cortex-bounty/Cargo.toml -p cortex-app-server --lib still hits the unrelated existing failure in tools::shell::tests::test_execute_shell on this machine, so the verification above skips that one pre-existing test and the remaining 39 library tests pass.

[greptile-apps]

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