refactor(core): move embedded system skills into codex-skills crate

[bolinfest]

Why

codex-core was carrying the embedded system-skill sample assets (and a build.rs that walks those files to register rerun triggers). Those assets change infrequently, but any change under codex-core still ties them to codex-core's build/cache lifecycle.

This change moves the embedded system-skills packaging into a dedicated codex-skills crate so it can be cached independently. That reduces unnecessary invalidation/rebuild pressure on codex-core when the skills bundle is the only thing that changes.

What Changed

• Added a new codex-rs/skills crate (codex-skills) with:
  • Cargo.toml
  • BUILD.bazel
  • build.rs to track skill asset file changes for Cargo rebuilds
  • src/lib.rs containing the embedded system-skills install/cache logic previously in codex-core
• Moved the embedded sample skill assets from codex-rs/core/src/skills/assets/samples to codex-rs/skills/src/assets/samples.
• Updated codex-rs/core/Cargo.toml to depend on codex-skills and removed codex-core's direct include_dir dependency.
• Removed codex-core's build.rs.
• Replaced codex-rs/core/src/skills/system.rs implementation with a thin re-export wrapper to keep existing codex-core call sites unchanged.
• Updated workspace manifests/lockfile (codex-rs/Cargo.toml, codex-rs/Cargo.lock) for the new crate.