add opencode runtime adapter and matrix fallback orchestration guidance

runtime/opencode/AGENTS.md

@@ -0,0 +1,28 @@
+# Mosaic Runtime Adapter — OpenCode
+
+Load and apply in this order:
+1. `~/.mosaic/STANDARDS.md`
+2. repo-local `AGENTS.md` (and `SOUL.md` when present)
+
+This file is an OpenCode adapter layer. It does not replace project guidance.
+
+## Orchestrator Capability Contract
+
+- OpenCode sessions may not expose a native subagent/background task primitive in all deployments.
+- When native subagent delegation is unavailable, use Mosaic's deterministic orchestrator rail:
+  - `~/.mosaic/bin/mosaic-orchestrator-matrix-cycle`
+- Keep repo task tracking in `docs/tasks.md` as the canonical human-readable state.
+- Keep machine transport/controller state in `.mosaic/orchestrator/`.
+
+## Fallback Delegation Mode (No Native Subagents)
+
+1. Bootstrap repo if needed:
+   - `~/.mosaic/bin/mosaic-bootstrap-repo /path/to/repo`
+2. Configure `.mosaic/orchestrator/config.json`:
+   - set `"enabled": true`
+   - set `"worker.command_template"` to your OpenCode worker command
+3. Queue work items in `.mosaic/orchestrator/tasks.json` (or via Matrix command ingestion).
+4. Run deterministic ticks:
+   - `~/.mosaic/bin/mosaic-orchestrator-matrix-cycle`
+
+This preserves rails enforcement (`quality_gates`) even without runtime-native subagent features.