[codex] isolate plugin/list from config serialization queue

[xli-oai]

Summary

• move plugin/list from the shared config read queue onto a dedicated plugin-list shared-read queue
• move plugin/read onto that same dedicated shared-read queue as well
• keep the existing scheduler behavior unchanged
• allow plugin list/read operations to proceed independently of config-family writes, accepting temporary stale or transient read errors during concurrent mutations

Validation

• just fmt
• cargo test -p codex-app-server-protocol