fix(repository): enforce Git mutation deadline

[Qiyuanqiii]

Summary

• enforce the documented two-minute Git mutation ceiling with a real child-process context deadline
• remove the ineffective post-exit elapsed-time check and kill attempt
• preserve a shorter caller cancellation/deadline instead of misclassifying the killed process as an ordinary Git failure
• surface the executor-owned limit as DEADLINE_EXCEEDED
• add fast injected-process tests for timeout, caller cancellation, and service-visible error classification

Root cause

runGit called command.Run() with the unbounded caller context and only compared elapsed time after the process had already exited. A hung Git process could therefore run forever; calling Process.Kill() after Run returned had no effect.

Verification

• go test -count=1 -v -timeout 60s ./internal/repository -run "TestMutationRunGit|TestMutationExecuteReportsDurationDeadline|TestMutationGitHelperProcess"
• normal mutation lifecycle regression
• go test -count=1 ./internal/application
• go vet ./internal/repository
• git diff --check