chore: address Homebrew formula hardening follow-ups from PR #742 review

[yvonnedevlinrh]

Context

Non-blocking observations from the PR #742 review that can be addressed together as a small hardening pass.

Items

1. Authenticate git clone in Homebrew workflow (LOW)

release.yml:146 — The git clone uses unauthenticated HTTPS. This works because homebrew-tap is public. If the repo were ever made private, the clone would fail before the push. Either add a comment documenting the public-repo assumption, or use the token for the clone as well.

2. Add retry backoff to tarball download (LOW)

release.yml:104-105 — The tarball download has --retry 3 but no backoff. Consider adding --retry-delay 5 to the curl invocation for CDN propagation delay.

3. Guard against double .git suffix in test script (LOW)

homebrew_test.sh:30 — Strip .git twice before re-adding to handle misconfigured remotes ending in .git.git.

4. Confirm GitHub App configuration on homebrew-tap (INFO)

Task 4.3 "Admin confirmation" is unchecked. Confirm the GitHub App is installed and scoped to complytime/homebrew-tap with Contents:write before the first release.

5. Align Go version references (INFO)

INSTALLATION.md says "Go 1.26 or later", AGENTS.md says "Go 1.25", go.mod says go 1.26.5. Align all references.

6. Add pre-flight remote branch check in test script (LOW)

The test script validates branch name format but doesn't verify the branch exists on the remote before handing off to Homebrew. This results in a confusing git clone error deep inside brew install instead of a clear early failure. Add a check after resolving the remote URL:

# After resolving REMOTE and BRANCH
if ! git ls-remote --heads "${REPO_URL}" "${BRANCH}" | grep -q .; then
    echo "ERROR: branch '${BRANCH}' not found on remote '${REPO_URL}'."
    echo "Push your branch first, or use 'gh pr checkout <number>' to check out the PR branch."
    exit 1
fi
Acceptance Criteria
- Items 1-3 addressed in release.yml and homebrew_test.sh
- Item 4 confirmed (checkbox or comment)
- Item 5: version references aligned
- Item 6: remote branch pre-flight check added to homebrew_test.sh