Refactor opencode into a shared workflow engine

[Copilot]

Opencode support was still wired into Go as a built-in engine. This moves it to declarative engine frontmatter so opencode behavior is maintained by a shared workflow instead.

• Shared engine definition

  • Added .github/workflows/shared/opencode.md with the opencode engine metadata, install/config/execution behavior, MCP config path, and provider-aware domains.
  • Updated smoke-opencode.md to import the shared engine.

• Go engine cleanup

  • Removed opencode-specific constants, default version/model env vars, registry wiring, validation branches, domain tables, and CLI help/completion entries.
  • Kept behavior-defined engine support generic through declarative frontmatter.

• Declarative domain support

  • Added behaviors.domains so shared behavior-defined engines can declare base and provider-specific network domains without Go changes.

imports:
  - shared/opencode.md

model: copilot/claude-sonnet-4.5

[pelikhan]

@copilot the compiler should populate folders/files with the information in opencode.md

[pelikhan]

@copilot only include the folders for the active agent.

[pelikhan]

@copilot Keep working, recompile

[Copilot]

@copilot Keep working, recompile

Recompiled and fixed the shared OpenCode schema validation gap in d2f0452. make recompile and make agent-report-progress both pass, and lock files are in sync.

[pelikhan]

@copilot Merge main and recompile.