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/io.py

@@ -0,0 +1,37 @@
+"""
+JSON file I/O utilities.
+
+Provides read_json and write_json as the single source of truth
+for JSON file operations across all Trellis scripts.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+
+def read_json(path: Path) -> dict | None:
+    """Read and parse a JSON file.
+
+    Returns None if the file doesn't exist, is invalid JSON, or can't be read.
+    """
+    try:
+        return json.loads(path.read_text(encoding="utf-8"))
+    except (FileNotFoundError, json.JSONDecodeError, OSError):
+        return None
+
+
+def write_json(path: Path, data: dict) -> bool:
+    """Write dict to JSON file with pretty formatting.
+
+    Returns True on success, False on error.
+    """
+    try:
+        path.write_text(
+            json.dumps(data, indent=2, ensure_ascii=False),
+            encoding="utf-8",
+        )
+        return True
+    except (OSError, IOError):
+        return False