[Feature Request] Implement weighted command history based on execution frequency

Problem: When typing / and pressing up-arrow, slash commands surface in recency order. Frequently used commands sink under one-off invocations, so the most-likely-next command takes multiple up-presses to reach.

Current behavior: Up-arrow walks the history backwards in chronological order — last-typed first, regardless of how often each command is actually used.

Proposed behavior: Maintain a per-command invocation counter; rank the up-arrow walk by frequency descending (with recency as a tiebreaker). The first up-arrow press surfaces the most-frequently-invoked command; subsequent presses walk down the frequency-sorted list.

Benefit: Faster recall of the commands you actually live in — /commit-push, /continue-roadmap, /fix-bug, etc. — without scrolling past one-off invocations. The cost of a single rare command is paid once; the savings on frequent commands compound every session.