refactor: unify identity/provisioned_key into key block

Replace the two mutually-exclusive repo keys (identity and
provisioned_key) with a single required key block. key.provider
is "static" (path to host SSH key) or "gitea" (deploy-key lifecycle
via provisioner_token env var, replacing token_env).

Internal fields: ManifestProvisionedKeyConfig → ManifestKeyConfig;
ProvisionedKey field removed from ManifestGitEntry; Key field added.
git_gate.py checks entry.Key.provider == "gitea" instead of
entry.ProvisionedKey is not None.

tests/unit/test_manifest_agent_git_user.py

@@ -112,7 +112,7 @@ class TestAgentGitUserOverlay(unittest.TestCase):
 class TestAgentGitUserRejections(unittest.TestCase):
     def test_agent_repos_dies_bottle_only(self):
         msg = _error_message(_manifest, agent_git={
-            "repos": {"r": {"url": "ssh://git@x/y.git", "identity": "/dev/null"}},
+            "repos": {"r": {"url": "ssh://git@x/y.git", "key": {"provider": "static", "path": "/dev/null"}}},
         })
         self.assertIn("git-gate.repos", msg)
         self.assertIn("bottle-only", msg)