feat: add skills extension scaffold (#25953)

## Disclaimer
This is only here for iteration purpose! Do not make any code rely on
this

## Why

Skills still live behind `codex-core` discovery and injection paths, but
the extension system needs an authority-aware home before that logic can
move. This adds that boundary without changing current skills behavior,
and keeps host, executor, and remote skills distinct so future
list/read/search flows do not collapse back to ambient local paths.

## What changed

- Add the `codex-skills-extension` workspace/Bazel crate under
`ext/skills`.
- Define the initial catalog, authority, provider, and turn-state types
for authority-bound skill packages and resources.
- Register placeholder thread/config/prompt/turn lifecycle contributors
plus host, executor, and remote provider aggregation points.
- Capture the remaining extraction work as TODOs, including the missing
extension API hooks needed for per-turn catalog construction and typed
skill injection.
- Keep plugins outside the runtime skills model: plugin-installed skills
are treated as materialized host-owned skill sources once available.

## Verification

- Not run locally.

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

@@ -0,0 +1,24 @@
+use codex_core::config::Config;
+
+use crate::catalog::SkillCatalog;
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub(crate) struct SkillsExtensionConfig {
+    pub(crate) include_instructions: bool,
+    pub(crate) bundled_skills_enabled: bool,
+}
+
+impl SkillsExtensionConfig {
+    pub(crate) fn from_config(config: &Config) -> Self {
+        Self {
+            include_instructions: config.include_skill_instructions,
+            bundled_skills_enabled: config.bundled_skills_enabled(),
+        }
+    }
+}
+
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+pub(crate) struct SkillsTurnState {
+    pub(crate) catalog: SkillCatalog,
+    pub(crate) entrypoints_injected: bool,
+}