chore: migrate from opencode to trellis 0.5.0-rc.6

- Remove legacy .opencode/ directory and configuration
- Update .trellis/ to v0.5.0-rc.6 structure
- Refactor scripts: modularize common/, remove multi_agent/
- Add new common modules: git.py, io.py, log.py, types.py, etc.
- Update workflow.md and AGENTS.md
- Archive completed migration tasks

AGENTS.md

@@ -59,16 +59,30 @@ When reading images, check whether the model has native multimodal capability fi
 
 These instructions are for AI assistants working in this project.
 
-Use the `/trellis:start` command when starting a new session to:
-- Initialize your developer identity
-- Understand current project context
-- Read relevant guidelines
+This project is managed by Trellis. The working knowledge you need lives under `.trellis/`:
 
-Use `@/.trellis/` to learn:
-- Development workflow (`workflow.md`)
-- Project structure guidelines (`spec/`)
-- Developer workspace (`workspace/`)
+- `.trellis/workflow.md` — development phases, when to create tasks, skill routing
+- `.trellis/spec/` — package- and layer-scoped coding guidelines (read before writing code in a given layer)
+- `.trellis/workspace/` — per-developer journals and session traces
+- `.trellis/tasks/` — active and archived tasks (PRDs, research, jsonl context)
 
-Keep this managed block so 'trellis update' can refresh the instructions.
+If a Trellis command is available on your platform (e.g. `/trellis:finish-work`, `/trellis:continue`), prefer it over manual steps. Not every platform exposes every command.
+
+If you're using Codex or another agent-capable tool, additional project-scoped helpers may live in:
+- `.agents/skills/` — reusable Trellis skills
+- `.codex/agents/` — optional custom subagents
+
+## Subagents
+
+- ALWAYS wait for every spawned subagent to reach a terminal status before yielding, acting on partial results, or spawning followups.
+  - On Codex, this means calling the `wait` tool with the subagent's thread id (requires `multi_agent_v2`). Do NOT infer completion from elapsed time.
+  - On Claude Code / OpenCode, this means awaiting the Task/agent tool result before continuing.
+- NEVER cancel or re-spawn a subagent that hasn't finished. If a subagent appears stuck, raise the wait timeout (Codex default 30s, max 1h) before judging it broken.
+- Spawn subagents automatically when:
+  - Parallelizable work (e.g., install + verify, npm test + typecheck, multiple tasks from plan)
+  - Long-running or blocking tasks where a worker can run independently
+  - Isolation for risky changes or checks
+
+Managed by Trellis. Edits outside this block are preserved; edits inside may be overwritten by a future `trellis update`.
 
 <!-- TRELLIS:END -->