[codex] Move thread naming to app server

[pakrym-oai]

Why

Thread names are app-server metadata now, backed by the thread store and sqlite state database. Keeping a core SetThreadName op plus a rollout thread_name_updated event made rename persistence live in the wrong layer and required historical replay support for an event that new app-server flows should not write.

What changed

• Removed Op::SetThreadName and EventMsg::ThreadNameUpdated from the core protocol and deleted the core handler path that appended rename events to rollouts.
• Updated app-server thread/name/set so both loaded and unloaded threads write through thread-store metadata and app-server emits thread/name/updated notifications.
• Updated local thread-store name metadata updates to write sqlite title metadata and the legacy thread-name index without appending rollout events.
• Removed state extraction and rollout handling for the deleted thread-name event.

Validation

• cargo test -p codex-app-server thread_name_updated_broadcasts
• cargo test -p codex-app-server thread_name_set_is_reflected_in_read_list_and_resume
• cargo test -p codex-thread-store update_thread_metadata_sets_name_on_active_rollout_and_indexes_name
• cargo test -p codex-state
• cargo check -p codex-mcp-server -p codex-rollout-trace
• just fix -p codex-app-server -p codex-thread-store -p codex-state -p codex-mcp-server -p codex-rollout-trace

Docs

No external documentation update is expected for this internal ownership change.

[wiltzius-openai]

still need to clean up app server codepaths to not touch the state DB directly but can be a followup