fix(bootstrap): add timezone guidance to USER.md template

Ensures model sees the hint to ask for timezone even after bootstrap
completes, preventing timezone from being permanently missed if user
skips the question during initial onboarding.

internal/bootstrap/templates/USER.md

@@ -5,7 +5,7 @@ _Learn about the person you're helping. Update this as you go._
 - **Name:**
 - **What to call them:**
 - **Pronouns:** _(optional)_
-- **Timezone:**
+- **Timezone:** _(ask when user mentions times/schedules if empty)_
 - **Notes:**
 
 ## Context