[good first issue] Add Aider setup guide

[GoDiao]

Goal

Add a copy-paste-ready setup guide for Aider in the README (or docs/agents/aider.md).

Required content

1. Command-line flags or env vars:
   • --openai-api-base http://localhost:8787/v1
   • --openai-api-key <your FREEWAY_API_KEY>
2. Model selection via --model flag using Freeway model IDs
3. Verification step

Acceptance criteria

• A new user can follow the guide and have Aider talking to Freeway in under 2 minutes
• The guide is linked from the main README

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.