fix(core): route Muse models to Meta prompt

[opencode-agent]

Summary

• route Muse-family model IDs through the Meta system prompt
• render the correct Muse Spark or Muse Glimmer name in identity and feedback text
• cover both model families and verify no template placeholder reaches the final prompt

Tests

• bun test test/plugin/system-prompt.test.ts
• bun typecheck

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