feat: track plugins mcps/apps and add plugin info to user_instructions (#13433)

### first half of changes, followed by #13510

Track plugin capabilities as derived summaries on `PluginLoadOutcome`
for enabled plugins with at least one skill/app/mcp.

Also add `Plugins` section to `user_instructions` injected on session
start. These introduce the plugins concept and list enabled plugins, but
do NOT currently include paths to enabled plugins or details on what
apps/mcps the plugins contain (current plan is to inject this on
@-mention). that can be adjusted in a follow up and based on evals.

### tests
Added/updated tests, confirmed locally that new `Plugins` section +
currently enabled plugins show up in `user_instructions`.

codex-rs/core/tests/suite/plugins.rs

@@ -169,6 +169,18 @@ async fn plugin_skills_append_to_instructions() -> Result<()> {
     let instructions_text = request_body["input"][1]["content"][0]["text"]
         .as_str()
         .expect("instructions text");
+    assert!(
+        instructions_text.contains("## Plugins"),
+        "expected plugins section present"
+    );
+    assert!(
+        instructions_text.contains("### Available plugins\n- `sample`"),
+        "expected enabled plugin list in instructions"
+    );
+    assert!(
+        instructions_text.contains("### How to use plugins"),
+        "expected plugin usage guidance heading"
+    );
     assert!(
         instructions_text.contains("## Skills"),
         "expected skills section present"
@@ -183,6 +195,10 @@ async fn plugin_skills_append_to_instructions() -> Result<()> {
         instructions_text.contains(&expected_path_str),
         "expected path {expected_path_str} in instructions"
     );
+    assert!(
+        instructions_text.find("## Plugins") < instructions_text.find("## Skills"),
+        "expected plugins section before skills section"
+    );
 
     Ok(())
 }