ci: configure git credentials for homebrew tap push

[hwbehrens]

Summary

The v0.6.0 homebrew tap update job now fails at git push — the previous PR (#16) fixed the git identity, but git push to the cloned tap repo can't authenticate because the GH_TOKEN (PAT) isn't picked up by plain git commands.

Root cause: gh repo clone authenticates via GH_TOKEN, but subsequent git push uses the default credential helper which has no credentials configured.

Fix: Set credential.helper globally to provide the PAT username/password to git, so git push authenticates with the same token.

Verification

• make verify passes (no Rust code changed)
• The commit + push sequence was the exact failure point in the v0.6.0 release run

Self-Assessment

Category	Max	Score	Notes
1) Correctness & Protocol Behavior	20	20	No protocol code changed
2) Security & Hardening	18	18	Credential helper is scoped to the CI job; PAT is already in the environment
3) Testing Quality & Coverage	20	18	CI-only change; validated on next release trigger
4) Performance & Resource Efficiency	12	12	No runtime code changed
5) Reliability & Robustness	10	10	Fails fast under set -euo pipefail
6) Maintainability & Code Health	12	12	Single-line addition, well-commented
7) Build/CI Hygiene	8	8	Fixes a CI failure

Score: 98/100