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

.claude/skills/trellis-meta/references/local-architecture/workspace-memory.md

@@ -0,0 +1,71 @@
+# Local Workspace Memory System
+
+`.trellis/workspace/` stores cross-session memory. Its purpose is to let AI and humans understand what happened before across different windows and different days.
+
+## Directory Structure
+
+```text
+.trellis/workspace/
+├── index.md
+└── <developer>/
+    ├── index.md
+    ├── journal-1.md
+    └── journal-2.md
+```
+
+| File | Purpose |
+| --- | --- |
+| `.trellis/.developer` | Current developer identity. |
+| `.trellis/workspace/index.md` | Global workspace overview. |
+| `.trellis/workspace/<developer>/index.md` | Session index for a developer. |
+| `.trellis/workspace/<developer>/journal-N.md` | Session journal. |
+
+## Developer Identity
+
+Run this the first time:
+
+```bash
+python3 ./.trellis/scripts/init_developer.py <name>
+```
+
+This creates `.trellis/.developer` and the corresponding workspace directory. The AI should not change developer identity casually; if the identity is wrong, first confirm who is using the current project.
+
+## Journal
+
+`journal-N.md` records completed or partially completed work from each session. By default, each journal holds about 2000 lines; after that it rotates to the next file.
+
+Common command for recording a session:
+
+```bash
+python3 ./.trellis/scripts/add_session.py \
+  --title "Session title" \
+  --summary "What changed" \
+  --commit "abc1234"
+```
+
+Planning or review work without a commit can also be recorded by using `--no-commit` or an empty commit value.
+
+## Relationship Between Workspace Memory And Tasks
+
+| System | What it stores |
+| --- | --- |
+| `.trellis/tasks/` | Requirements, design, research, and state for a specific task. |
+| `.trellis/workspace/` | Work records across tasks and sessions. |
+| `.trellis/spec/` | Engineering knowledge preserved as long-term conventions. |
+
+If information is only useful for the current task, put it in the task directory.  
+If information describes what happened in the current session, put it in the workspace journal.  
+If information should be followed every time code is written in the future, put it in spec.
+
+## Local Customization Points
+
+| Need | Edit location |
+| --- | --- |
+| Change maximum journal lines | `max_journal_lines` in `.trellis/config.yaml`. |
+| Change session auto-commit message | `session_commit_message` in `.trellis/config.yaml`. |
+| Change session content format | `.trellis/scripts/add_session.py`. |
+| Change how workspace is displayed in context | `.trellis/scripts/common/session_context.py`. |
+
+## AI Usage Rules
+
+The AI should not treat workspace as the only source of truth. When resuming a task, read the current task first, then use workspace for background. After a task is complete, record important process notes in workspace; if long-term rules emerged, update spec.