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_git_gate.py

@@ -284,7 +284,7 @@ class TestPrepare(unittest.TestCase):
             "bottles": {"dev": {"git-gate": {"repos": {
                 "foo": {
                     "url": "ssh://git@github.com/didericis/foo.git",
-                    "identity": "/dev/null",
+                    "key": {"provider": "static", "path": "/dev/null"},
                 },
             }}}},
             "agents": {"demo": {"skills": [], "prompt": "", "bottle": "dev"}},