Add plugin to pool cli [DOCS/FEATURE]

[maxkaustav]

To add custom plugins to pool like ponytail

[CryptoJones]

I prepared a README-only documentation patch for this request.

It adds:

• project and user Agent Skills discovery paths
• $ selection, /skills refresh/list behavior, and automatic activation
• guidance for multi-skill packages such as Ponytail
• a clear boundary between portable Agent Skills and host-specific hooks, status lines, or plugin settings

Upstream currently has pull requests disabled, so I could not open one directly. The patch is available here:

• Compare against main
• Commit 11adeef

Validation: Markdown renders successfully, all README links resolve, and the change introduces no new markdownlint findings.