Add OpenRouter model generator

[nicopujia]

Summary

Add an OpenRouter generator that pulls from GET /api/v1/models and generates OpenRouter model TOMLs.

The generator supports:

• --dry-run to preview changes without writing files
• --new-only to create missing models without updating existing ones
• --delete-orphaned to remove local model files that are no longer present in the OpenRouter API

Testing

bun install
bun openrouter:generate --dry-run
bun openrouter:generate --dry-run --delete-orphaned
bun validate

Optional write-mode checks:

bun openrouter:generate --new-only
bun validate