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/git.py

@@ -0,0 +1,31 @@
+"""
+Git command execution utility.
+
+Single source of truth for running git commands across all Trellis scripts.
+"""
+
+from __future__ import annotations
+
+import subprocess
+from pathlib import Path
+
+
+def run_git(args: list[str], cwd: Path | None = None) -> tuple[int, str, str]:
+    """Run a git command and return (returncode, stdout, stderr).
+
+    Uses UTF-8 encoding with -c i18n.logOutputEncoding=UTF-8 to ensure
+    consistent output across all platforms (Windows, macOS, Linux).
+    """
+    try:
+        git_args = ["git", "-c", "i18n.logOutputEncoding=UTF-8"] + args
+        result = subprocess.run(
+            git_args,
+            cwd=cwd,
+            capture_output=True,
+            text=True,
+            encoding="utf-8",
+            errors="replace",
+        )
+        return result.returncode, result.stdout, result.stderr
+    except Exception as e:
+        return 1, "", str(e)