Binding an empty repo (no commits) made `git clone` land an unborn HEAD with no
`origin/main`. `add_task_worktree` / `create_result_branch` then fell back to
base_ref=HEAD, and `git worktree add ... HEAD` failed with "invalid reference:
HEAD". The agent silently degraded to "executing on repo root without git push",
so the result branch never reached the remote.
Fix: after clone (and after refreshing an existing checkout), detect an empty
repo via `git rev-parse --verify HEAD` and plant one empty baseline commit on the
configured base branch. Worktree/branch creation and push then work normally and
the result branch is created on the (previously empty) remote.
Adds scripts/test-git-empty-repo.py covering the empty-repo path; existing
scripts/test-git-workflow.py still passes (no regression). Verified end-to-end
against a real empty GitHub repo: result branch pushed successfully.
Impact: Swarm agent git layer only (agent/git_operations.py). No change to
Manager<->Swarm contract, callbacks, billing, secret_ref, or audit.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
