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/ext/extension-api/src/contributors/mcp.rs

@@ -93,8 +93,8 @@ pub enum McpServerContribution {
         selection_order: usize,
         config: Box<McpServerConfig>,
     },
-    /// Adds connector IDs declared by a plugin selected for this thread.
-    SelectedPluginConnectors {
+    /// Records a plugin selected for this thread and any connector IDs it declares.
+    SelectedPluginPackage {
         plugin_id: String,
         plugin_display_name: String,
         connector_ids: Vec<String>,