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/.gitignore

@@ -4,6 +4,9 @@
 # Current task pointer (each dev works on different task)
 .current-task
 
+# Session/window scoped runtime state
+.runtime/
+
 # Ralph Loop state file
 .ralph-state.json