Add a "User-Only" Mode for Skills

[anexplore]

Proposal

It would be useful if Codex CLI supported a user-only mode for skills, similar to the behavior available in Claude Code.

Currently, skills enabled under /skills can potentially be selected and used automatically by the model. However, there are some skills that users may want to keep available without allowing the model to invoke them automatically.

I suggest adding another skill activation mode, for example:

Enabled — the model can automatically use the skill when appropriate.
User-only — the skill is available, but can only be invoked explicitly by the user through a / command or similar manual mechanism.
Disabled — the skill cannot be used.

For example, a user might configure a skill as:

Skill A: Enabled
Skill B: User-only
Skill C: Disabled

In this case, Codex could automatically use Skill A, but it should never invoke Skill B on its own. Skill B would only be executed when the user explicitly requests it, for example:

/skill-b

Why this would be useful

Some skills may:

perform expensive or time-consuming operations;
modify files or external systems;
have side effects that users want tighter control over;
be useful only in very specific situations;
introduce unnecessary context or tool-selection overhead when available for automatic invocation.

A user-only mode would allow users to keep these skills installed and easily accessible without having to repeatedly enable and disable them.

It would also provide a clearer distinction between:

skills available to the model and skills available only as explicit user commands.

This would make /skills more flexible and give users better control over when specific skills are invoked.