Slash command autocomplete should prioritize by frequency, then user-defined, then built-ins

Problem

I have a plugin-defined /launch skill that I use many times per day. The recently added built-in /loop command now takes priority over /launch in slash command autocomplete. When I type /l, I get /loop first — a command I've never used — instead of my custom skills starting with L.

Adding new built-in commands shouldn't break existing user workflows.

Proposed Fix

Sort slash command autocomplete by:

1. Frequency of use — commands the user actually invokes should float to the top
2. User/plugin-defined skills — at equal frequency, user skills should rank above built-ins
3. Built-in commands — lowest priority

This way autocomplete adapts to how each user actually works, and new built-in commands don't displace established workflows.