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_compose.py

@@ -51,7 +51,7 @@ def _manifest(*, supervise: bool, with_git: bool, with_egress: bool) -> Manifest
         bottle["git-gate"] = {"repos": {
             "upstream": {
                 "url": "ssh://git@example.com:22/x/y.git",
-                "identity": "/etc/hostname",  # any existing file
+                "key": {"provider": "static", "path": "/etc/hostname"},
             },
         }}
     if with_egress: