fix: add compaction todo preserver hook

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/compaction-todo-preserver/index.test.ts

@@ -0,0 +1,74 @@
+import { describe, expect, it, mock } from "bun:test"
+import type { PluginInput } from "@opencode-ai/plugin"
+import { createCompactionTodoPreserverHook } from "./index"
+
+const updateMock = mock(async () => {})
+
+mock.module("opencode/session/todo", () => ({
+  Todo: {
+    update: updateMock,
+  },
+}))
+
+type TodoSnapshot = {
+  id: string
+  content: string
+  status: "pending" | "in_progress" | "completed" | "cancelled"
+  priority?: "low" | "medium" | "high"
+}
+
+function createMockContext(todoResponses: TodoSnapshot[][]): PluginInput {
+  let callIndex = 0
+  return {
+    client: {
+      session: {
+        todo: async () => {
+          const current = todoResponses[Math.min(callIndex, todoResponses.length - 1)] ?? []
+          callIndex += 1
+          return { data: current }
+        },
+      },
+    },
+    directory: "/tmp/test",
+  } as PluginInput
+}
+
+describe("compaction-todo-preserver", () => {
+  it("restores todos after compaction when missing", async () => {
+    //#given
+    updateMock.mockClear()
+    const sessionID = "session-compaction-missing"
+    const todos = [
+      { id: "1", content: "Task 1", status: "pending", priority: "high" },
+      { id: "2", content: "Task 2", status: "in_progress", priority: "medium" },
+    ]
+    const ctx = createMockContext([todos, []])
+    const hook = createCompactionTodoPreserverHook(ctx)
+
+    //#when
+    await hook.capture(sessionID)
+    await hook.event({ event: { type: "session.compacted", properties: { sessionID } } })
+
+    //#then
+    expect(updateMock).toHaveBeenCalledTimes(1)
+    expect(updateMock).toHaveBeenCalledWith({ sessionID, todos })
+  })
+
+  it("skips restore when todos already present", async () => {
+    //#given
+    updateMock.mockClear()
+    const sessionID = "session-compaction-present"
+    const todos = [
+      { id: "1", content: "Task 1", status: "pending", priority: "high" },
+    ]
+    const ctx = createMockContext([todos, todos])
+    const hook = createCompactionTodoPreserverHook(ctx)
+
+    //#when
+    await hook.capture(sessionID)
+    await hook.event({ event: { type: "session.compacted", properties: { sessionID } } })
+
+    //#then
+    expect(updateMock).not.toHaveBeenCalled()
+  })
+})