feat(opencode): add optional runtime overlay

[Danigm-dev]

Summary

Adds OpenCode as an optional Sim capability without changing the default Sim deployment path or base UX.

This PR introduces:

• the OpenCode workflow block, tools, API routes, and lib/opencode client/service wiring
• async dropdown/combobox support needed by the OpenCode block selectors
• an optional OpenCode runtime with local/prod Docker Compose overlays
• minimal docs and env examples for opt-in setup
• a configurable OPENCODE_REPOSITORY_ROOT contract so Sim can also target compatible external OpenCode deployments

The default Sim experience remains unchanged:

• docker-compose.local.yml is unchanged
• docker-compose.prod.yml is unchanged
• the OpenCode block stays hidden unless NEXT_PUBLIC_OPENCODE_ENABLED=true

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation
• Other: ___________

Testing

• timeout 90s bunx vitest run blocks/blocks.test.ts from apps/sim
• bunx vitest run lib/opencode/service.test.ts app/api/opencode/repos/route.test.ts app/api/tools/opencode/prompt/route.test.ts from apps/sim
• docker compose -f docker-compose.local.yml -f docker-compose.opencode.local.yml config
• env OPENCODE_SERVER_PASSWORD=change-me docker compose -f docker-compose.prod.yml -f docker-compose.opencode.yml config
• Manual validation:
  • Sim only: base UX unchanged and OpenCode hidden by default
  • Sim + OpenCode local overlay: OpenCode service starts, healthcheck passes, repo catalog loads, and prompts execute from the OpenCode block

Reviewer focus:

• Confirm the default Sim path is unchanged when OpenCode is not enabled
• Review the OpenCode runtime contract and OPENCODE_REPOSITORY_ROOT handling for external deployments
• Review shared dropdown/combobox async-loading changes for regressions

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

Screenshots/Videos

Not included. The OpenCode block is hidden by default and only appears when NEXT_PUBLIC_OPENCODE_ENABLED=true. Manual validation covered both states and a successful prompt execution flow.