Implement v1 skills extension prompt injection

[jif-oai]

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.