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_extends.py

@@ -116,8 +116,8 @@ class TestExtendsGitMerge(unittest.TestCase):
     """git-gate.user overlays by field; git-gate.repos merges by upstream
     host, with child entries replacing duplicate hosts."""
 
-    _GIT_ENTRY_A = {"url": "ssh://git@host-a/a.git", "identity": "/dev/null"}
-    _GIT_ENTRY_B = {"url": "ssh://git@host-b/b.git", "identity": "/dev/null"}
+    _GIT_ENTRY_A = {"url": "ssh://git@host-a/a.git", "key": {"provider": "static", "path": "/dev/null"}}
+    _GIT_ENTRY_B = {"url": "ssh://git@host-b/b.git", "key": {"provider": "static", "path": "/dev/null"}}
 
     def test_child_git_repos_merge_with_parent(self):
         m = _build(
@@ -131,7 +131,7 @@ class TestExtendsGitMerge(unittest.TestCase):
         self.assertEqual(["a", "b"], names)
 
     def test_child_git_repo_replaces_same_host(self):
-        replacement = {"url": "ssh://git@host-a/replacement.git", "identity": "/dev/null"}
+        replacement = {"url": "ssh://git@host-a/replacement.git", "key": {"provider": "static", "path": "/dev/null"}}
         m = _build(
             base={"git-gate": {"repos": {"a": self._GIT_ENTRY_A}}},
             child={