Add skills announce usage config

[mrubens]

Summary

Adds a new Codex config flag:

[skills]
announce_usage = false

When unset, the effective value defaults to true, preserving current behavior. When set to false, Codex still injects the automatic skills instructions block but omits only this guidance line:

- Announce which skill(s) you're using and why (one short line). If you skip an obvious skill, say why.

The setting is included in the effective runtime config, preserved through config locking/replay, and threaded into both absolute-path and alias-based skills rendering.

Request

Enhancement request: add a config escape hatch for skill-use announcement guidance while keeping automatic skills instructions and explicit skill injection behavior unchanged.

No public issue link was provided for this draft PR.

Testing

• just write-config-schema
• just fmt
• cargo test -p codex-core-skills
• cargo test -p codex-core
  One unrelated timeout occurred in agent::control::tests::resume_thread_subagent_restores_stored_nickname_and_role.
• cargo test -p codex-core agent::control::tests::resume_thread_subagent_restores_stored_nickname_and_role
• just fix -p codex-core
• just fix -p codex-core-skills

Rename follow-up:

• just write-config-schema
• just fmt
• cargo test -p codex-core skill_announce_usage
• cargo test -p codex-core-skills render_body