`git init` without `-b` inherits the ambient `init.defaultBranch` config,
which is `master` unless the host has configured otherwise, so `pkgmgr
repos create` produced master-based repositories. On top of that, the
push fallback renamed an already-correct `main` branch back to `master`
whenever the initial push failed (missing remote, auth error).
`init()` now requires an explicit `initial_branch` and runs
`git init -b <branch>`; the bootstrapper passes `main` and no longer
falls back to `master` on push failure.
`resolve_base_branch()` keeps its main -> master fallback, since it reads
existing repositories that may legitimately use master.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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