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/plugins/render.rs

@@ -6,7 +6,7 @@ use crate::plugins::PluginCapabilitySummary;
 
 #[cfg(test)]
 pub(crate) fn render_plugins_section(plugins: &[PluginCapabilitySummary]) -> Option<String> {
-    AvailablePluginsInstructions::from_plugins(plugins).map(|instructions| instructions.render())
+    (!plugins.is_empty()).then(|| AvailablePluginsInstructions.render())
 }
 
 pub(crate) fn render_explicit_plugin_instructions(