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/core/src/state/service.rs

@@ -40,7 +40,7 @@ use tokio_util::sync::CancellationToken;
 
 pub(crate) struct SessionServices {
     /// The latest manager; callers retain an owned handle while performing MCP I/O.
-    pub(crate) mcp_connection_manager: ArcSwap<McpConnectionManager>,
+    pub(crate) mcp_connection_manager: Arc<ArcSwap<McpConnectionManager>>,
     pub(crate) mcp_startup_cancellation_token: Mutex<CancellationToken>,
     pub(crate) unified_exec_manager: UnifiedExecProcessManager,
     #[cfg_attr(not(unix), allow(dead_code))]