fix: add Pioneer Devstral Small 2 route

[opencode-agent]

Summary

• add shared Devstral Small 2 metadata
• add Pioneer's currently served devstral-small-2 route
• record Pioneer pricing and served limits

Evidence

• Pioneer's public model API lists the route as active with text and image inputs
• the API reports a 256,000-token input limit, 131,072-token output limit, and $0.10/$0.30 per million input/output tokens
• Mistral's launch and model card establish the shared model identity and capabilities

Validation

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

Closes #4130

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

[github-actions]

No actionable findings.