fix: Detect non-standard default branch names

[charlesvien]

Problem

Git setup fails with fatal: invalid reference main when a repo's default branch isn't main or master, because detection only checked those two names and callers silently fell back to hardcoded "main".

Changes

1. Add init.defaultBranch config and current HEAD as fallbacks in detectDefaultBranch()
2. Fix detectDefaultBranchWithFallback() to try HEAD before hardcoding "main"
3. Fix workspace service .catch(() => "main") to use current branch
4. Fix git service GitHub API fallback to return empty instead of guessing "main"
5. Add unit tests for all default branch detection strategies

How did you test this?

Manually + added unit tests

[charlesvien]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

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