Add day-of-week to /usage token reset date

Feature Request

The /usage command displays token reset dates as just month and day (e.g. "Jan 30"), but doesn't include the day of the week. Adding the day of the week (e.g. "Fri, Jan 30") would make it easier to quickly understand when limits reset relative to your schedule.

Proposed Solutions (either would work)

Option A: Add day-of-week to /usage output directly

Simply include the day of the week in the reset date display. For example:

Current: Resets Jan 30
Proposed: Resets Fri, Jan 30

Option B: Allow extensibility of built-in command output

Provide a hook event (e.g. PostBuiltinCommand) or other extension point that lets users customize or augment the output of built-in slash commands like /usage. This would be a more general solution that enables other customizations as well.

Context

There's currently no way to modify built-in command output — hooks, skills, plugins, and MCP servers can only create new commands, not extend existing ones. Option A is the simpler ask; Option B would be more broadly useful.