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.

bot_bottle/manifest.py

@@ -56,7 +56,7 @@ from .manifest_egress import (
     ManifestEgressConfig,
     ManifestEgressRoute,
 )
-from .manifest_git import ManifestGitEntry, ManifestGitUser, parse_git_gate_config
+from .manifest_git import ManifestGitEntry, ManifestGitUser, ManifestKeyConfig, parse_git_gate_config
 from .manifest_schema import BOTTLE_KEYS
 
 # Re-export everything that callers currently import from this module.
@@ -64,6 +64,7 @@ __all__ = [
     "ManifestError",
     "ManifestGitEntry",
     "ManifestGitUser",
+    "ManifestKeyConfig",
     "ManifestAgentProvider",
     "EGRESS_AUTH_SCHEMES",
     "ManifestEgressRoute",