Implement v1 skills extension prompt injection (#26167)

## Why

The skills extension needs a real turn-time path before host, executor,
or remote skills can be routed through it. The previous code was mostly
a placeholder catalog/provider sketch, so there was no bounded
available-skills fragment, no source-owned `SKILL.md` read, and no place
for warnings or per-turn selection state to live.

This PR makes `ext/skills` the authority-preserving flow for listing
candidate skills and injecting only explicitly selected main prompts,
without adding more of that logic to `codex-core`.

## What changed

- Expands catalog entries with `main_prompt`, display path, short
description, dependency metadata, enabled/prompt visibility flags, and
authority/package-aware read requests.
- Replaces the placeholder `providers/*` modules with
`SkillProviderSource` and `SkillProviders`, routing list/read/search
calls by source kind and surfacing provider failures as warnings.
- Adds bounded available-skills rendering and `SKILL.md` main-prompt
truncation before the fragments enter model context.
- Resolves explicit skill selections from structured `UserInput::Skill`,
skill-file mentions, `skill://...` paths, and plain `$skill` text
mentions, then reads selected prompts through their owning provider.
- Stores mutable per-thread skills config and per-turn
catalog/selection/warning state.
- Adds `install_with_providers` so tests and future host wiring can
supply concrete providers.

## Testing

- Not run locally.
- Added `codex-rs/ext/skills/tests/skills_extension.rs` coverage for
available-catalog injection, selected prompt injection through the
owning provider, and prompt-hidden skills that remain invokable.

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

@@ -1,6 +1,8 @@
 use codex_core::config::Config;
+use std::sync::Mutex;
 
 use crate::catalog::SkillCatalog;
+use crate::catalog::SkillCatalogEntry;
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub(crate) struct SkillsExtensionConfig {
@@ -17,8 +19,37 @@ impl SkillsExtensionConfig {
     }
 }
 
+#[derive(Debug)]
+pub(crate) struct SkillsThreadState {
+    config: Mutex<SkillsExtensionConfig>,
+}
+
+impl SkillsThreadState {
+    pub(crate) fn new(config: SkillsExtensionConfig) -> Self {
+        Self {
+            config: Mutex::new(config),
+        }
+    }
+
+    pub(crate) fn config(&self) -> SkillsExtensionConfig {
+        self.config
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner)
+            .clone()
+    }
+
+    pub(crate) fn set_config(&self, config: SkillsExtensionConfig) {
+        *self
+            .config
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner) = config;
+    }
+}
+
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub(crate) struct SkillsTurnState {
     pub(crate) catalog: SkillCatalog,
-    pub(crate) entrypoints_injected: bool,
+    pub(crate) selected_entries: Vec<SkillCatalogEntry>,
+    pub(crate) warnings: Vec<String>,
+    pub(crate) main_prompts_injected: bool,
 }