fix: handle empty homebrew-mumei repo in update-homebrew job

[AkiraTamai]

fix: handle empty homebrew-mumei repo in update-homebrew job

Summary

Fixes the update-homebrew job failing with fatal: couldn't find remote ref refs/heads/main when the mumei-lang/homebrew-mumei repository is empty (has no branches).

Root cause: The actions/checkout@v4 action assumes the target repo has a default branch. The homebrew-mumei repo was empty, so fetching refs/heads/main failed repeatedly.

What changed:

1. Checkout step: Replaced actions/checkout@v4 with a manual git clone that falls back to git init -b main + git remote add when the repo is empty.
2. Push step: Replaced peter-evans/create-pull-request@v6 with a manual git commit + git push origin HEAD:main. This pushes the formula directly to main instead of creating a PR, which is simpler for automated formula updates and works correctly for both empty and existing repos.

Updates since last revision

• Commit 4 (9701ceb): Handle empty homebrew-mumei repo — replaces actions/checkout + peter-evans/create-pull-request with manual git commands that gracefully handle repos with no branches.

Previous commits (already merged into develop via earlier PRs):

• Commit 1 (055ce06): macos-13 → macos-15-intel for x86_64-apple-darwin
• Commit 2 (f48ed6d): RUSTFLAGS Z3 linker path for musl (superseded by commit 3)
• Commit 3 (2bc4885): Removed musl target entirely

Review & Testing Checklist for Human

• Verify HOMEBREW_TAP_TOKEN secret has push access to mumei-lang/homebrew-mumei. The workflow now pushes directly to main instead of creating a PR, so the token needs write/push permissions (not just PR creation).
• Check that 2>/dev/null on git clone won't mask auth errors. If HOMEBREW_TAP_TOKEN is invalid or expired, the clone will silently fail and fall back to git init, which will then fail at the push step. This is acceptable (the push step will surface the auth error) but worth being aware of.
• Behavioral change: direct push vs PR. Previously the workflow created a PR on homebrew-mumei via peter-evans/create-pull-request. Now it pushes directly to main. Confirm this is acceptable for your workflow — there will be no review step for formula updates.
• Tag a new release (e.g. v0.2.11) after merging to trigger the release workflow and verify the update-homebrew job succeeds. This cannot be tested locally.

Test Plan

1. Merge this PR into develop, then merge develop into main
2. Push a new version tag (e.g. v0.2.11)
3. Verify all 4 build targets pass (these should be unaffected)
4. Verify the update-homebrew job completes successfully
5. Check that mumei-lang/homebrew-mumei has a main branch with the generated formula

Notes

• Failed job (v0.2.10): job 66066089039 — fatal: couldn't find remote ref refs/heads/main
• Previous CI failures: v0.2.4, v0.2.5, v0.2.6, v0.2.8, v0.2.9
• Related PRs: fix: switch reqwest to rustls-tls to fix OpenSSL cross-compilation failures #39 (rustls-tls), fix: use native ARM runner for aarch64 build, add fail-fast: false #41 (ARM runner + fail-fast), fix: use macos-13 for x86_64-apple-darwin, build static Z3 for musl #43 (macos-13 + static Z3 for musl)
• Requested by: @AkiraTamai
• Devin session: https://app.devin.ai/sessions/c6a0819f07cb40f0858159ae49b2019e

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✅ Devin Review: No Issues Found

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