Allow configuring the number of slash command suggestions in the picker

Problem

The slash command picker displays a fixed number of suggestions. Users with many skills, plugins, and custom commands installed see only a small fraction of available options at a time. There is no setting, environment variable, or config key to control this.

Proposal

Add a configurable setting (via /config or settings.json) to control how many slash command suggestions the picker displays. Something like:

{
  "slashCommandSuggestionCount": 15
}

A reasonable default (current behavior) with the option to increase would work well.

Why this matters

Power users who install multiple plugins and define custom skills can easily accumulate 50+ slash commands. The current fixed picker height makes discovery and selection harder than it needs to be, especially when commands share prefixes or similar names. The recent improvements to character highlighting and description wrapping help, but showing more results at once would compound those gains.

Alternatives considered

• Scrolling through the full list works but is slow with many entries
• Typing a more specific query narrows results, but requires knowing what you're looking for