Clarify worker setup for custom API keys

[Bortlesboat]

Summary

• explain that custom API keys are configured through the Worker, not an in-app settings screen
• fix the README grep example to search for the actual placeholder Worker URL used in the Swift files

Why

• issue Running out of credit - how can I use my own codex or claude code API key? #27 shows contributors are looking for an in-app key entry flow that does not exist yet
• the current README tells people to grep for clicky-proxy, but the checked-in Swift files use your-worker-name.your-subdomain.workers.dev

Refs #27

Verification

• the README grep example now matches the two live Swift references:
  • leanring-buddy/CompanionManager.swift
  • leanring-buddy/AssemblyAIStreamingTranscriptionProvider.swift