modes() returns None for WorkspaceWrite sessions

[codefromthecrypt]

Problem

session/new and session/load responses omit the modes field for trusted projects using the default auto (WorkspaceWrite) preset.

modes() matches the runtime sandbox policy against presets using PartialEq. The auto preset has writable_roots: vec![] (via new_workspace_write_policy()), but config loading always appends memories_root to writable_roots for any WorkspaceWrite policy. The derived PartialEq compares all fields, so the match fails.

handle_set_mode works fine if called directly. The issue is only that modes() can't identify the current mode to advertise.

Repro

1. Start codex-acp with a trusted project
2. Send session/new
3. Response has sessionId and models but no modes

Related: #57, #172