feat: support product-scoped plugins. (#15041)

1. Added SessionSource::Custom(String) and --session-source.
  2. Enforced plugin and skill products by session_source.
  3. Applied the same filtering to curated background refresh.

codex-rs/tui/src/resume_picker.rs

@@ -164,7 +164,7 @@ async fn run_session_picker(
                 PAGE_SIZE,
                 request.cursor.as_ref(),
                 request.sort_key,
-                INTERACTIVE_SESSION_SOURCES,
+                INTERACTIVE_SESSION_SOURCES.as_slice(),
                 Some(provider_filter.as_slice()),
                 request.default_provider.as_str(),
                 /*search_term*/ None,