Make plugin guidance react to environment readiness (#30223)

## Why

Generic plugin guidance is currently emitted only with initial context
from host plugin state. An executor-selected plugin can become available
later in the same turn, making its skills and tools usable without ever
telling the model how plugin capabilities should be used.

## What

- project every ready selected plugin package, including skill-only
plugins
- carry plugin availability with the exact MCP runtime projection while
preserving MCP manager reuse when servers and connectors are unchanged
- move generic plugin guidance from the static initial-context path into
persisted World State
- recognize legacy and retained plugin fragments so resume and
compaction do not duplicate guidance

## Testing

- `just test -p codex-mcp-extension`
- `just test -p codex-core plugins_instructions`
- `just test -p codex-core
plugin_availability_change_reuses_the_mcp_manager`
- `just test -p codex-app-server --test all selected_capabilit`

codex-rs/core/src/thread_manager_tests.rs

@@ -699,11 +699,11 @@ async fn start_thread_seeds_extension_data_for_mcp_and_lifecycle_contributors()
             .collect::<std::collections::BTreeMap<_, _>>()
     };
     assert_eq!(
-        selected_servers(&first_resolved),
+        selected_servers(&first_resolved.config),
         std::collections::BTreeMap::from([("selected-a".to_string(), "env-a".to_string())])
     );
     assert_eq!(
-        selected_servers(&second_resolved),
+        selected_servers(&second_resolved.config),
         std::collections::BTreeMap::from([("selected-b".to_string(), "env-b".to_string())])
     );
 }