test: update test suite for git-gate manifest redesign (PRD 0047)

- fixtures.py: fixture_with_git_dict uses git-gate.repos + url/identity/host_key
- test_manifest_git: rewrite to use git-gate.repos; replace duplicate-name
  test (names = dict keys, always unique) with two-repos-different-hosts test
- test_manifest_git_user: _manifest → git-gate.user; update error message assertions
- test_manifest_agent_git_user: git → git-gate throughout; repos rejection test
- test_manifest_extends: git.remotes/git.user → git-gate.repos/git-gate.user
- test_provision_git: IP test updated — no host alias, single insteadOf
- test_compose: git.remotes → git-gate.repos + new field names
- test_docker_provision_git_user: git.user → git-gate.user
- test_git_gate: inline manifest dict updated to git-gate.repos
- test_smolmachines_provision: git_json → git_gate_json; remove _remote_host

tests/unit/test_docker_provision_git_user.py

@@ -30,7 +30,7 @@ def _plan(*, git_user: dict | None = None,
           stage_dir: Path | None = None) -> DockerBottlePlan:
     bottle_json: dict = {}
     if git_user is not None:
-        bottle_json["git"] = {"user": git_user}
+        bottle_json["git-gate"] = {"user": git_user}
     manifest = Manifest.from_json_obj({
         "bottles": {"dev": bottle_json},
         "agents": {"demo": {"skills": [], "prompt": "", "bottle": "dev"}},