Allow user-defined aliases for built-in slash commands (the internal `aliases` array is not user-configurable)

Summary

Built-in slash commands already carry an aliases array internally, but there is no way for a user to add one. Please expose it in settings.

In 2.1.273 the /clear entry is:

type:"local",name:"clear",description:"Start a new session with empty context; previous session stays on disk (resumable with /resume)",argumentHint:"[name]",aliases:["reset","new"],supportsNonInteractive:!0,...

So /reset and /new work, but only because they are hardcoded. I wanted /end for the same thing and there is no supported way to get it.

What does not work today

• A custom command at ~/.claude/commands/end.md is a prompt template. It injects text for the model; it cannot invoke a built-in that resets client session state.
• Keybindings do not cover it. The chat:* actions are attentionDown, attentionUp, cancel, clearInput, clearScreen, cycleMode, cycleProactivity, externalEditor, fastMode, imagePaste, killAgents, modelPicker, newline, queueSubmit, stash, submit, thinkingToggle, undo, workflowKeywordToggle. chat:clearScreen redraws the terminal and does not drop context.
• Hooks do not cover it. hookSpecificOutput carries context injection and permission decisions, nothing that resets a session.

Suggested approach

A settings key that feeds the same alias resolution the built-in aliases array already uses:

{
  "commandAliases": {
    "end": "clear"
  }
}

Reject an alias that collides with an existing command or built-in alias rather than silently shadowing it.

Prior issues

• #32785 — same request for /compact, closed as not planned after inactivity, now locked.
• #14576 — /alias command, closed as not planned, larger scope.
• #23589 — shorthand aliases for plugin commands, closed as duplicate.

Environment

• Version: 2.1.273
• Platform: darwin 25.6.0