fix(trail): require branch push to origin before creating a trail

Make delivering the branch to origin a precondition for 'entire trail create'. Previously the push was gated on the CLI having created the branch locally, so 'git checkout -b foo && entire trail create --branch foo' bound the trail without ever pushing foo. The server then had no remote branch to anchor to and backfilled it at the base tip, producing local/remote/work drift while still printing 'Created trail'.

Now the branch is always pushed (hard-failing on auth/non-fast-forward errors) and the create request sends branch_action=link so the server attaches the delivered branch instead of backfilling at base.