move plugin/skill instructions into dev msg and reorder (#14609)

Move the general `Apps`, `Skills` and `Plugins` instructions blocks out
of `user_instructions` and into the developer message, with new `Apps ->
Skills -> Plugins` order for better clarity.

Also wrap those sections in stable XML-style instruction tags (like
other sections) and update prompt-layout tests/snapshots. This makes the
tests less brittle in snapshot output (we can parse the sections), and
it consolidates the capability instructions in one place.

#### Tests
Updated snapshots, added tests.

`<AGENTS_MD>` disappearing in snapshots is expected: before this change,
the wrapped user-instructions message was kept alive by `Skills`
content. Now that `Skills` and `Plugins` are in the developer message,
that wrapper only appears when there is real
project-doc/user-instructions content.

---------

Co-authored-by: Charley Cunningham <ccunningham@openai.com>

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

@@ -61,6 +61,7 @@ fn summary_with_prefix(summary: &str) -> String {
 
 fn context_snapshot_options() -> ContextSnapshotOptions {
     ContextSnapshotOptions::default()
+        .strip_capability_instructions()
         .render_mode(ContextSnapshotRenderMode::KindWithTextPrefix { max_chars: 64 })
 }