#19308: fix(commands): use session model for native arg menus

## Summary Native /think menus were using the config default provider/model when building arg menus, so xhigh was omitted even when the active session model supported it. This uses the current session’s model context for Telegram/Discord/Slack native command menus. ## Changes - Pass provider/model into resolveCommandArgMenu and its choice resolution - Resolve provider/model from the session store for native command menus - Add a commands-registry test for explicit provider/model context - Update Slack menu mock signature ## Testing pnpm vitest run src/auto-reply/commands-registry.test.ts src/slack/monitor/slash.test.ts