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/config.yaml

@@ -31,3 +31,29 @@ max_journal_lines: 2000
 #     - "echo 'Task finished'"
 #   after_archive:
 #     - "echo 'Task archived'"
+
+#-------------------------------------------------------------------------------
+# Monorepo / Packages
+#-------------------------------------------------------------------------------
+
+# Declare packages for monorepo projects.
+# Trellis auto-detects workspaces during `trellis init`, but you can also
+# configure them manually here.
+#
+# packages:
+#   frontend:
+#     path: packages/frontend
+#   backend:
+#     path: packages/backend
+#   docs:
+#     path: docs-site
+#     type: submodule
+#   # For polyrepo / meta-repo layouts (independent .git in each subdir),
+#   # mark the package with `git: true`. The runtime treats it as an
+#   # independent repository for things like git-context display.
+#   webapp:
+#     path: ./webapp
+#     git: true
+
+# Default package used when --package is not specified.
+# default_package: frontend