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/provider.rs

@@ -4,8 +4,10 @@ use std::sync::Arc;
 
 mod executor;
 mod host;
+mod orchestrator;
 
 use codex_core_skills::HostLoadedSkills;
+use codex_mcp::McpResourceClient;
 use codex_protocol::capabilities::SelectedCapabilityRoot;
 
 use crate::catalog::SkillAuthority;
@@ -18,6 +20,7 @@ use crate::catalog::SkillSearchResult;
 
 pub use executor::ExecutorSkillProvider;
 pub use host::HostSkillProvider;
+pub use orchestrator::OrchestratorSkillProvider;
 
 #[derive(Clone, Debug)]
 pub struct SkillListQuery {
@@ -26,7 +29,8 @@ pub struct SkillListQuery {
     pub host: Option<Arc<HostLoadedSkills>>,
     pub include_host_skills: bool,
     pub include_bundled_skills: bool,
-    pub include_remote_skills: bool,
+    pub include_orchestrator_skills: bool,
+    pub mcp_resources: Option<Arc<McpResourceClient>>,
 }
 
 #[derive(Clone, Debug)]
@@ -35,6 +39,7 @@ pub struct SkillReadRequest {
     pub package: SkillPackageId,
     pub resource: SkillResourceId,
     pub host: Option<Arc<HostLoadedSkills>>,
+    pub mcp_resources: Option<Arc<McpResourceClient>>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]