Update README with setup guide, providers, and examples

[nev-offload]

Problem

The README needs to be updated to reflect the current state of the project, including OpenRouter support and all supported providers.

Content needed

1. Quick start guide — minimal steps to get the action running
2. Supported providers — list all 8 providers with example model strings
3. Configuration reference — all inputs, secrets, and environment variables
4. Trigger examples — workflow YAML for each supported trigger type:
   • Issue opened
   • PR opened
   • Comment mention
   • Scheduled cron
   • Manual dispatch
5. OpenRouter setup — specific instructions for using OpenRouter (free models!)
6. .openclaw.yml config — document the repo config file format (once implemented)
7. Architecture diagram — how the action connects to the gateway
8. Troubleshooting — common errors and fixes

Priority

P2 — Important for adoption but not blocking development.

[nev-offload]

✅ Completed

Enhanced README with comprehensive documentation:

Added sections:

• Development guide (local setup, building, type checking)
• Building & Publishing workflow
• Release process with versioning guidance
• Local testing instructions
• Repository testing guide
• Detailed caching explanation
• Self-hosting & forking guide
• One-click setup suggestions
• Expanded contributing guidelines

The README now covers all aspects from development to deployment and self-hosting.