feat(hooks): add todo-description-override hook to enforce atomic todo format

Override TodoWrite description via tool.definition hook to require
WHERE/WHY/HOW/RESULT in each todo title and enforce 1-3 tool call
granularity.

src/hooks/todo-description-override/index.test.ts

@@ -0,0 +1,40 @@
+import { describe, it, expect } from "bun:test"
+import { createTodoDescriptionOverrideHook } from "./hook"
+import { TODOWRITE_DESCRIPTION } from "./description"
+
+describe("createTodoDescriptionOverrideHook", () => {
+  describe("#given hook is created", () => {
+    describe("#when tool.definition is called with todowrite", () => {
+      it("#then should override the description", async () => {
+        const hook = createTodoDescriptionOverrideHook()
+        const output = { description: "original description", parameters: {} }
+
+        await hook["tool.definition"]({ toolID: "todowrite" }, output)
+
+        expect(output.description).toBe(TODOWRITE_DESCRIPTION)
+      })
+    })
+
+    describe("#when tool.definition is called with non-todowrite tool", () => {
+      it("#then should not modify the description", async () => {
+        const hook = createTodoDescriptionOverrideHook()
+        const output = { description: "original description", parameters: {} }
+
+        await hook["tool.definition"]({ toolID: "bash" }, output)
+
+        expect(output.description).toBe("original description")
+      })
+    })
+
+    describe("#when tool.definition is called with TodoWrite (case-insensitive)", () => {
+      it("#then should not override for different casing since OpenCode sends lowercase", async () => {
+        const hook = createTodoDescriptionOverrideHook()
+        const output = { description: "original description", parameters: {} }
+
+        await hook["tool.definition"]({ toolID: "TodoWrite" }, output)
+
+        expect(output.description).toBe("original description")
+      })
+    })
+  })
+})