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/config/mod.rs

@@ -52,6 +52,7 @@ use codex_config::types::TuiNotificationSettings;
 use codex_config::types::TuiPetAnchor;
 use codex_config::types::UriBasedFileOpener;
 use codex_config::types::WindowsSandboxModeToml;
+use codex_core_plugins::PluginLoadOutcome;
 use codex_core_plugins::PluginsConfigInput;
 use codex_exec_server::ExecutorFileSystem;
 use codex_exec_server::LOCAL_FS;
@@ -1528,6 +1529,14 @@ impl Config {
     ) -> McpConfig {
         let plugins_input = self.plugins_config_input();
         let loaded_plugins = plugins_manager.plugins_for_config(&plugins_input).await;
+        self.to_mcp_config_with_loaded_plugins(&loaded_plugins, additional_plugin_registrations)
+    }
+
+    pub(crate) fn to_mcp_config_with_loaded_plugins(
+        &self,
+        loaded_plugins: &PluginLoadOutcome,
+        additional_plugin_registrations: impl IntoIterator<Item = McpServerRegistration>,
+    ) -> McpConfig {
         let mut catalog = ResolvedMcpCatalog::builder();
         for (plugin_order, plugin) in loaded_plugins
             .plugins()