skills: make backend plugin skills invocable without an executor (#27387)

## Why

#27198 made the extension-owned `codex_apps` MCP connection the hosted
plugin runtime, but its `mcp/skill` resources still bypassed the skills
extension. App-server could list and read those resources through
generic MCP APIs, but a thread with no selected environment did not
expose them in the model's skills catalog or load their `SKILL.md`
through `$skill`.

Hosted skills should stay remote while using the same typed catalog,
source authority, deduplication, bounded contextual catalog, and
selected-skill prompt injection as host and executor skills. They should
not be downloaded or exposed as ambient filesystem paths.

## What changed

- Add a session-scoped `McpResourceClient` over the replaceable MCP
connection manager so resource list/read calls follow startup and
refresh replacements.
- Add a `BackendSkillProvider` that pages `codex_apps` resources,
accepts bounded and validated `mcp/skill` entries, and reads a selected
skill's `SKILL.md` through the same MCP connection.
- Register the remote provider in app-server and include it in the
skills catalog even when a thread has no selected capability roots or
executor.
- Contribute hosted skill metadata through the bounded
`AvailableSkillsInstructions` developer-context path, exclude remote
entries from per-turn catalog injection, and classify `<skills>`
messages as contextual developer content so rollback can trim and
rebuild them correctly.

## Testing

- Extend the app-server MCP resource integration test with
`environments: []` to exercise two-page discovery, filter a
non-`mcp/skill` resource, verify the escaped developer catalog entry and
user-role `<skill>` fragment containing the fetched `SKILL.md`, and
preserve generic MCP resource reads.
- Add core event-mapping coverage that classifies `<skills>` developer
messages as contextual history.

codex-rs/ext/skills/src/render.rs

@@ -1,7 +1,4 @@
-use codex_core_skills::render_available_skills_body;
-use codex_extension_api::ContextualUserFragment;
-use codex_protocol::protocol::SKILLS_INSTRUCTIONS_CLOSE_TAG;
-use codex_protocol::protocol::SKILLS_INSTRUCTIONS_OPEN_TAG;
+use codex_core::context::AvailableSkillsInstructions;
 use codex_utils_string::take_bytes_at_char_boundary;
 
 use crate::catalog::SkillCatalog;
@@ -11,30 +8,9 @@ const MAX_MAIN_PROMPT_BYTES: usize = 8_000;
 pub(crate) const MAX_SKILL_NAME_BYTES: usize = 256;
 pub(crate) const MAX_SKILL_PATH_BYTES: usize = 1_024;
 
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub(crate) struct AvailableSkillsFragment {
-    body: String,
-}
-
-impl ContextualUserFragment for AvailableSkillsFragment {
-    fn role(&self) -> &'static str {
-        "developer"
-    }
-
-    fn markers(&self) -> (&'static str, &'static str) {
-        Self::type_markers()
-    }
-
-    fn body(&self) -> String {
-        self.body.clone()
-    }
-
-    fn type_markers() -> (&'static str, &'static str) {
-        (SKILLS_INSTRUCTIONS_OPEN_TAG, SKILLS_INSTRUCTIONS_CLOSE_TAG)
-    }
-}
-
-pub(crate) fn available_skills_fragment(catalog: &SkillCatalog) -> Option<AvailableSkillsFragment> {
+pub(crate) fn available_skills_fragment(
+    catalog: &SkillCatalog,
+) -> Option<AvailableSkillsInstructions> {
     let mut total_bytes = 0usize;
     let mut omitted = 0usize;
     let mut skill_lines = Vec::new();
@@ -68,9 +44,7 @@ pub(crate) fn available_skills_fragment(catalog: &SkillCatalog) -> Option<Availa
         ));
     }
 
-    Some(AvailableSkillsFragment {
-        body: render_available_skills_body(&[], &skill_lines),
-    })
+    Some(AvailableSkillsInstructions::from_skill_lines(skill_lines))
 }
 
 fn render_skill_line(name: &str, description: &str, path: &str) -> String {