feat(tui): remove /provider alias for /model (#20358)

/model is the canonical command; /provider was a redundant alias that
dispatched to the same ModelPicker overlay. Drop the alias, the regex
branch in useCompletion, and the alias-coverage test.

ui-tui/src/app/slash/commands/session.ts

@@ -62,7 +62,6 @@ export const sessionCommands: SlashCommand[] = [
 
   {
     help: 'change or show model',
-    aliases: ['provider'],
     name: 'model',
     run: (arg, ctx) => {
       if (ctx.session.guardBusySessionSwitch('change models')) {