ci(homebrew): handle existing upstream remote and main base

[willsarg]

Fix Homebrew publish failure when upstream remote already exists after fork clone, and target upstream main branch with master fallback for fetch.

[github-actions]

PR intake checks found warnings (non-blocking)

Fast safe checks found advisory issues. CI lint/test/build gates still enforce merge quality.

• Missing required PR template sections: ## Summary, ## Validation Evidence (required), ## Security Impact (required), ## Privacy and Data Hygiene (required), ## Rollback Plan (required)
• Incomplete required PR template fields: summary problem, summary why it matters, summary what changed, validation commands, security risk/mitigation, privacy status, rollback plan

Action items:

1. Complete required PR template sections/fields.
2. Remove tabs, trailing whitespace, and merge conflict markers from added lines.
3. Re-run local checks before pushing:
   • ./scripts/ci/rust_quality_gate.sh
   • ./scripts/ci/rust_strict_delta_gate.sh
   • ./scripts/ci/docs_quality_gate.sh

Run logs: https://github.com/zeroclaw-labs/zeroclaw/actions/runs/22275210822

Detected blocking line issues (sample):

• none

Detected advisory line issues (sample):

• none

Workflow files changed in this PR:

• .github/workflows/pub-homebrew-core.yml

Reminder: workflow changes require owner approval via CI Required Gate.

[Copilot]

Pull request overview

This PR updates the Homebrew CI workflow to handle cases where the upstream remote already exists when cloning a fork, and implements dynamic detection of the upstream repository's default branch (main vs master). The changes address failures in the Homebrew publish workflow when working with forked repositories.

Changes:

• Add logic to handle existing upstream remote by using set-url instead of add
• Dynamically detect whether upstream uses "main" or "master" branch
• Update branch checkout and PR creation to use detected branch name

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[Copilot]

The PR base branch is hardcoded to "main" but the earlier logic detects whether the upstream uses "main" or "master" and stores it in the upstream_ref variable. However, upstream_ref is a local variable in the patch_formula step and is not available in this later push-and-PR step. This creates an inconsistency: the branch is created based on the detected upstream_ref (line 146), but the PR always targets "main" regardless of what was detected.

If the upstream repository still uses "master", this will cause the PR creation to fail because the base branch won't exist or won't match the branch that was actually checked out.