add opencode runtime adapter and matrix fallback orchestration guidance

guides/orchestrator.md

@@ -27,7 +27,7 @@ The orchestrator **cold-starts** on any project with just a review report locati
 **The orchestrator ONLY:**
 - Reads/writes `docs/tasks.md`
 - Reads/writes `docs/orchestrator-learnings.json`
-- Spawns workers via the Task tool for ALL code changes
+- Delegates ALL code changes to workers (native subagent tool when available, otherwise Mosaic matrix rail)
 - Parses worker JSON results
 - Commits task tracking updates (tasks.md, learnings)
 - Outputs status reports and handoff messages
@@ -40,6 +40,23 @@ Spawn a worker instead. No exceptions. No "quick fixes."
 - Wait for at least one worker to complete before spawning more
 - This optimizes token usage and reduces context pressure
 
+## Delegation Mode Selection
+
+Choose one delegation mode at session start:
+
+1. **Native subagent mode** (preferred when runtime supports it)
+2. **Matrix rail mode** (fallback when native subagents/background tasks are unavailable)
+
+Matrix rail mode commands:
+
+```bash
+~/.mosaic/bin/mosaic-orchestrator-matrix-cycle
+~/.mosaic/bin/mosaic-orchestrator-run --poll-sec 10
+```
+
+In Matrix rail mode, keep `docs/tasks.md` as canonical project tracking and use
+`.mosaic/orchestrator/` for deterministic worker dispatch state.
+
 ---
 
 ## Bootstrap Templates
@@ -251,7 +268,9 @@ git push
    - All done? → Report success, run final retrospective, STOP
    - Some blocked? → Report deadlock, STOP
 5. Update tasks.md: status=in-progress, agent={identifier}, started_at={now}
-6. Spawn worker agent (Task tool) with task details
+6. Delegate worker task:
+   - native mode: spawn worker agent via runtime subagent/task primitive
+   - matrix mode: enqueue/consume task in `.mosaic/orchestrator/tasks.json` and run `mosaic-orchestrator-matrix-cycle`
 7. Wait for worker completion
 8. Parse worker result (JSON)
 9. **Variance check**: Calculate (actual - estimate) / estimate × 100