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/scripts/common/__init__.py

@@ -75,8 +75,18 @@ from .paths import (
     count_lines,
     get_current_task,
     get_current_task_abs,
+    normalize_task_ref,
+    resolve_task_ref,
     set_current_task,
     clear_current_task,
     has_current_task,
     generate_task_date_prefix,
 )
+
+from .active_task import (
+    ActiveTask,
+    clear_active_task,
+    resolve_active_task,
+    resolve_context_key,
+    set_active_task,
+)