Improve extension READMEs with .env.example files

[matthallett1]

What

Add .env.example files to each of the 6 extensions so users know exactly which environment variables to set.

Why

Currently the extension READMEs describe env vars in prose, but having a concrete .env.example file in each extension folder is clearer and more copy-paste friendly.

What's needed

For each extension in extensions/:

• Create a .env.example with the required variables and placeholder values
• Example:

  SUPABASE_URL=https://your-project-ref.supabase.co
  SUPABASE_SERVICE_ROLE_KEY=your-service-role-key
  

• The meal-planning extension has two servers, so it may need two example files or a note

Difficulty

Beginner — documentation only, no code changes.

[justfinethanku]

Covered by PR #81.