Bypass inherited Git credential helpers

[yvette-carlisle]

Summary

• clear inherited Git credential helpers for routed GitHub credential commands
• keep Decodex askpass as the owned auth path for default-branch fetch/sync
• update the default-branch sync credential test to cover the empty helper

Verification

• cargo test -p decodex default_branch_git_commands_use_routed_noninteractive_credentials
• cargo test -p decodex git_credentials
• cargo make fmt
• cargo make lint-fix
• git diff --check