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/render.rs

@@ -0,0 +1,90 @@
+use codex_core_skills::render_available_skills_body;
+use codex_extension_api::ContextualUserFragment;
+use codex_protocol::protocol::SKILLS_INSTRUCTIONS_CLOSE_TAG;
+use codex_protocol::protocol::SKILLS_INSTRUCTIONS_OPEN_TAG;
+
+use crate::catalog::SkillCatalog;
+
+const MAX_AVAILABLE_SKILLS_CHARS: usize = 8_000;
+const MAX_MAIN_PROMPT_CHARS: usize = 40_000;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) struct AvailableSkillsFragment {
+    body: String,
+}
+
+impl ContextualUserFragment for AvailableSkillsFragment {
+    fn role(&self) -> &'static str {
+        "developer"
+    }
+
+    fn markers(&self) -> (&'static str, &'static str) {
+        Self::type_markers()
+    }
+
+    fn body(&self) -> String {
+        self.body.clone()
+    }
+
+    fn type_markers() -> (&'static str, &'static str) {
+        (SKILLS_INSTRUCTIONS_OPEN_TAG, SKILLS_INSTRUCTIONS_CLOSE_TAG)
+    }
+}
+
+pub(crate) fn available_skills_fragment(catalog: &SkillCatalog) -> Option<AvailableSkillsFragment> {
+    let mut total_chars = 0usize;
+    let mut omitted = 0usize;
+    let mut skill_lines = Vec::new();
+
+    for entry in catalog
+        .entries
+        .iter()
+        .filter(|entry| entry.enabled && entry.prompt_visible)
+    {
+        let description = entry
+            .short_description
+            .as_deref()
+            .unwrap_or(entry.description.as_str());
+        let line = render_skill_line(entry.name.as_str(), description, entry.rendered_path());
+        let next_chars = total_chars.saturating_add(line.chars().count());
+        if next_chars > MAX_AVAILABLE_SKILLS_CHARS {
+            omitted = omitted.saturating_add(1);
+            continue;
+        }
+        total_chars = next_chars;
+        skill_lines.push(line);
+    }
+
+    if skill_lines.is_empty() {
+        return None;
+    }
+    if omitted > 0 {
+        let skill_word = if omitted == 1 { "skill" } else { "skills" };
+        skill_lines.push(format!(
+            "- {omitted} additional {skill_word} omitted from this bounded skills list."
+        ));
+    }
+
+    Some(AvailableSkillsFragment {
+        body: render_available_skills_body(&[], &skill_lines),
+    })
+}
+
+fn render_skill_line(name: &str, description: &str, path: &str) -> String {
+    if description.is_empty() {
+        format!("- {name}: (file: {path})")
+    } else {
+        format!("- {name}: {description} (file: {path})")
+    }
+}
+
+pub(crate) fn truncate_main_prompt_contents(contents: &str) -> (String, bool) {
+    let mut chars = 0usize;
+    for (index, _) in contents.char_indices() {
+        if chars == MAX_MAIN_PROMPT_CHARS {
+            return (contents[..index].to_string(), true);
+        }
+        chars = chars.saturating_add(1);
+    }
+    (contents.to_string(), false)
+}