fix(core): restore v2 model prompts

[opencode-agent]

Summary

• copy the legacy provider/model-specific system prompts into core for native V2 runner use
• select the provider-family baseline when the effective agent does not provide a system override
• keep built-in build agent from shadowing model-specific prompts and update the V2 parity checklist

Tests

• bun test test/session-runner-system-prompt.test.ts test/agent.test.ts test/session-runner.test.ts (packages/core)
• bun typecheck (packages/core)
• pre-push bun turbo typecheck