refactor(agent): move placeholder env injection into agent_provision_plan

The has_provider_auth check and egress-placeholder injection were
duplicated in both backends. Move them into agent_provision_plan so
the provisioner owns that decision entirely:

- Replace has_provider_auth: bool param with manifest_egress_routes,
  compute has_provider_auth internally from the route roles.
- Inject CLAUDE_CODE_OAUTH_TOKEN=egress-placeholder inside the plan
  when has_provider_auth, alongside the existing nonessential-traffic
  vars. Backends no longer touch the placeholder env.
- Remove placeholder_env from AgentProviderRuntime; expose
  placeholder_env_for() for print_util's hide-from-summary logic.

Assisted-by: Claude Code

bot_bottle/backend/print_util.py

@@ -9,7 +9,7 @@ from __future__ import annotations
 
 from typing import Sequence
 
-from ..agent_provider import runtime_for
+from ..agent_provider import placeholder_env_for
 from ..log import info
 
 
@@ -41,5 +41,5 @@ def visible_agent_env_names(
     think a real key is entering the agent, so hide only the active
     provider-owned placeholder.
     """
-    hidden = {runtime_for(agent_provider_template).placeholder_env}
+    hidden = {placeholder_env_for(agent_provider_template)}
     return sorted({name for name in env_names if name and name not in hidden})