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

.trellis/tasks/archive/2026-05/00-join-zl-q/task.json

@@ -0,0 +1,26 @@
+{
+  "id": "00-join-zl-q",
+  "name": "00-join-zl-q",
+  "title": "Joining: Onboard to this Trellis project (zl-q)",
+  "description": "Onboard a new developer to an existing Trellis project: learn the workflow, conventions, and find assigned work",
+  "status": "completed",
+  "dev_type": "docs",
+  "scope": null,
+  "package": null,
+  "priority": "P1",
+  "creator": "zl-q",
+  "assignee": "zl-q",
+  "createdAt": "2026-05-06",
+  "completedAt": "2026-05-06",
+  "branch": null,
+  "base_branch": null,
+  "worktree_path": null,
+  "commit": null,
+  "pr_url": null,
+  "subtasks": [],
+  "children": [],
+  "parent": null,
+  "relatedFiles": [],
+  "notes": "Generated by trellis init for a new developer joining an existing Trellis project",
+  "meta": {}
+}