fix(materialize/renderers): stop consuming REGISTRY for OpenCode/Copilot installs

[davidarce]

Summary

Pairs with the catalog PR that deletes `REGISTRY.opencode.md` / `REGISTRY.copilot.md` and slims `ORCHESTRATOR.{opencode,copilot}.md` to be the single-source orchestrator playbook.

OpenCode and Copilot have a primary `sdd-orchestrator` agent the user explicitly selects (custom agent in `opencode.json` / `.agent.md` in `.github/agents/`). That selection IS the consent to use the workflow. Until now, the renderer was also:

1. Prepending `REGISTRY.{opencode,copilot}.md` to the synthesised orchestrator prompt
2. Capturing the same content into `r.registryContents` for later injection into the ambient instructions file (AGENTS.md / copilot-instructions.md)

Both paths leaked workflow-specific rules into non-workflow sessions (ambient catalog) and duplicated / contradicted the orchestrator's own playbook (prompt prepend). They were a Claude/Factory pattern applied where it does not fit.

This change removes both consumers for OpenCode and Copilot. Claude / Factory / Codex paths are unchanged.

Changes

• `opencode.go` (RenderWorkflow): skip `captureRegistryContent` for the workflow registry; pass `registryPath=""` to `synthesizeAgents`. Comment block at the call site explains why (primary-agent installs do not need REGISTRY).
• `opencode.go` (buildOrchestratorEntry): signature unchanged for backward compatibility — non-empty `registryPath` is still honoured if a future caller passes one — but the canonical caller now passes "". Doc-comment updated.
• `copilot.go`: parallel changes — skip `captureRegistryContent`; pass `registryPath=""` to `installOrchestratorAgent`. Same explanatory comment.

Tests

• `TestOpenCodeRenderer_InstallWorkflow_RegistryInjectedIntoCatalog` → renamed to `TestOpenCodeRenderer_InstallWorkflow_RegistryNotCaptured`. Asserts `RegistryContents()` does NOT contain the workflow name. Negative assertion (no loose REGISTRY.md in skills dir) preserved.
• `TestCopilotRenderer_InstallWorkflow_RegistryInjectedIntoCatalog` → renamed to `TestCopilotRenderer_InstallWorkflow_RegistryNotCaptured`. Same flip.
• All other tests pass unchanged. `go test ./...` clean.

Compatibility

The change is forward-only: a workspace that materialised before this change still has the old REGISTRY-flavoured prompt in `opencode.json` / `.agent.md` until the next `devrune sync`, at which point the new prompt is rendered (REGISTRY content drops out, slim ORCHESTRATOR replaces it).

A workspace running this renderer against an old catalog (one that still ships `REGISTRY.{opencode,copilot}.md`) simply ignores those files — no error, no leak. A workspace running the old renderer against the new catalog (which deletes them) falls back to `REGISTRY.md` (generic Claude flavour) for the registry path, but reads an empty file capture, so the impact is harmless.

Pairing PR

Catalog: https://github.com/davidarce/devrune-starter-catalog/pull/new/fix/sdd-opencode-copilot-slim-orchestrator

Recommended order: merge catalog first, then this PR.

Test plan

• `go test ./...` passes (already verified locally).
• Merge both PRs, then `devrune sync` against an OpenCode workspace.
• Verify `opencode.json` orchestrator prompt is the new slim ORCHESTRATOR alone (no Evaluation Gate, no "Your Role" defensive prose).
• Verify `AGENTS.md` no longer carries the SDD orchestrator section.
• Same for a Copilot install (`.github/agents/sdd-orchestrator.agent.md` body, `.github/copilot-instructions.md` ambient).