Make thread store process-scoped (#19474)

- Build one app-server process ThreadStore from startup config and share
it with ThreadManager and CodexMessageProcessor.
- Remove per-thread/fork store reconstruction so effective thread config
cannot switch the persistence backend.
- Add params to ThreadStore create/resume for specifying thread
metadata, since otherwise the metadata from store creation would be used
(incorrectly).

codex-rs/core/src/session/tests/guardian_tests.rs

@@ -729,7 +729,7 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
     let mcp_manager = Arc::new(McpManager::new(Arc::clone(&plugins_manager)));
     let skills_watcher = Arc::new(SkillsWatcher::noop());
     let thread_store = Arc::new(codex_thread_store::LocalThreadStore::new(
-        codex_rollout::RolloutConfig::from_view(&config),
+        codex_thread_store::LocalThreadStoreConfig::from_config(&config),
     ));
 
     let CodexSpawnOk { codex, .. } = Codex::spawn(CodexSpawnArgs {