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/docker/prepare.py

@@ -168,19 +168,6 @@ def resolve_plan(
     # never lands on argv or in env_file) goes into one dict. Nothing
     # mutates the host os.environ.
     forwarded_env: dict[str, str] = dict(resolved.forwarded)
-    # Some provider CLIs refuse to start without *some* credential
-    # env var even when egress will strip + re-inject the real
-    # Authorization header. For those providers, auth_role names the
-    # route marker that enables a non-secret placeholder env. Codex is
-    # intentionally absent here: it should use its device/ChatGPT login
-    # state, and an OPENAI_API_KEY placeholder would force API-key auth.
-    has_provider_auth = any(
-        provider_runtime.auth_role
-        and provider_runtime.auth_role in r.roles
-        for r in egress_manifest_routes(bottle)
-    )
-    if has_provider_auth and provider_runtime.placeholder_env:
-        forwarded_env[provider_runtime.placeholder_env] = "egress-placeholder"
     _write_env_file(resolved, env_file)
     prompt_file.write_text(agent.prompt)
 
@@ -191,7 +178,7 @@ def resolve_plan(
         state_dir=agent_dir,
         guest_home=os.environ.get("BOT_BOTTLE_CONTAINER_HOME", "/home/node"),
         forward_host_credentials=provider.forward_host_credentials,
-        has_provider_auth=has_provider_auth,
+        manifest_egress_routes=egress_manifest_routes(bottle),
         host_env=dict(os.environ),
     )
     guest_env = dict(agent_provision.guest_env)