fix: add Pioneer Magistral Medium route

[opencode-agent]

Summary

• add Pioneer's currently served magistral-medium route
• inherit shared Magistral Medium metadata
• record Pioneer pricing, output limit, reasoning efforts, and reasoning side channel

Evidence

• Pioneer's public model API lists the route as active
• the API reports a 128,000-token input limit, 64,000-token output limit, and $2.00/$5.00 per million input/output tokens
• Pioneer metadata exposes low, medium, and high reasoning efforts

Validation

• bun validate
• bun packages/core/script/sync-models.ts pioneer --dry-run --no-issues (0 updates; route no longer reported missing)

Closes #4131

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

[github-actions]

No actionable findings.