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/codex-mcp/src/lib.rs

@@ -3,6 +3,9 @@ pub use connection_manager::tool_is_model_visible;
 pub use elicitation::ElicitationReviewRequest;
 pub use elicitation::ElicitationReviewer;
 pub use elicitation::ElicitationReviewerHandle;
+pub use resource_client::McpResourceClient;
+pub use resource_client::McpResourcePage;
+pub use resource_client::McpResourceReadResult;
 pub use rmcp_client::MCP_SANDBOX_STATE_META_CAPABILITY;
 pub use runtime::McpRuntimeContext;
 pub use runtime::SandboxState;
@@ -58,6 +61,7 @@ pub(crate) mod codex_apps;
 pub(crate) mod connection_manager;
 pub(crate) mod elicitation;
 pub(crate) mod mcp;
+mod resource_client;
 pub(crate) mod rmcp_client;
 pub(crate) mod runtime;
 pub(crate) mod server;