Support per-skill disable for plugin-contributed skills

Some plugins contribute too many skills to the session-visible skill list.

For example, the vercel plugin adds a large number of vercel:* skills, but I only want to keep vercel:agent-browser. Right now the config appears to support only enabling/disabling the whole plugin, not selecting individual skills.

This creates two problems:

1. unnecessary noise in the visible skill list
2. pressure on the model-visible skill budget, which can trigger warnings like:

Some enabled skills were not included in the model-visible skills list for this session.

It would be helpful to support per-skill filtering for plugin-contributed skills, for example via an allowlist or denylist in config.

Example desired behavior:

[plugins."vercel@openai-curated"]
enabled = true
enabled_skills = ["agent-browser"]

This would let users keep the few skills they actually use without disabling the whole plugin.