fix(todo): make Todo id field optional for OpenCode beta compatibility

- Make id field optional in all Todo interfaces (TodoInfo, Todo, TodoItem)
- Fix null-unsafe comparisons in todo-sync.ts to handle missing ids
- Add test case for todos without id field preservation
- All tests pass and typecheck clean

src/shared/opencode-message-dir.ts

@@ -0,0 +1,25 @@
+import { existsSync, readdirSync } from "node:fs"
+import { join } from "node:path"
+import { MESSAGE_STORAGE } from "../tools/session-manager/constants"
+
+export function getMessageDir(sessionID: string): string | null {
+  if (!existsSync(MESSAGE_STORAGE)) return null
+
+  const directPath = join(MESSAGE_STORAGE, sessionID)
+  if (existsSync(directPath)) {
+    return directPath
+  }
+
+  try {
+    for (const dir of readdirSync(MESSAGE_STORAGE)) {
+      const sessionPath = join(MESSAGE_STORAGE, dir, sessionID)
+      if (existsSync(sessionPath)) {
+        return sessionPath
+      }
+    }
+  } catch {
+    return null
+  }
+
+  return null
+}