[FEATURE] Load skills on-demand instead of listing all in every session

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

## Problem
Every session injects the full list of available skills into the system prompt,
consuming tokens even when most skills are never used in that session.

Proposed Solution

Proposed Solution

Only list skills on-demand — either:

1. A /skills command that lists available skills when needed, or
2. Skills announced only when a relevant keyword is detected, or
3. A per-plugin opt-in flag like "lazyLoad": true in plugin settings

## Impact
Currently 40–50 skill entries are injected every turn. With on-demand loading,
this would drop to ~0 unless explicitly requested.

## Workaround
Disabling unused plugins reduces the list, but core plugins (superpowers)
still inject skills regardless.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_