feat(core): restore Modal model discovery

[opencode-agent]

Summary

• add the missing built-in Modal provider plugin
• fetch the authenticated workspace model catalog from the Modal proxy /models endpoint
• replace static template IDs with live workspace deployment IDs while preserving model metadata and applying live capabilities, limits, pricing, compatibility, and reasoning variants
• refresh the catalog when the Modal connection changes and hide unusable static models when discovery fails

Testing

• bun test test/plugin/provider-modal.test.ts
• bun test test/plugin (232 passing)
• bun typecheck

Requested by: @rekram1-node (Aiden Cline via Slack)