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/index.ts

@@ -52,3 +52,4 @@ export { createWriteExistingFileGuardHook } from "./write-existing-file-guard";
 export { createHashlineReadEnhancerHook } from "./hashline-read-enhancer";
 export { createJsonErrorRecoveryHook, JSON_ERROR_TOOL_EXCLUDE_LIST, JSON_ERROR_PATTERNS, JSON_ERROR_REMINDER } from "./json-error-recovery";
 export { createReadImageResizerHook } from "./read-image-resizer"
+export { createTodoDescriptionOverrideHook } from "./todo-description-override"

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

@@ -0,0 +1,28 @@
+export const TODOWRITE_DESCRIPTION = `Use this tool to create and manage a structured task list for tracking progress on multi-step work.
+
+## Todo Format (MANDATORY)
+
+Each todo title MUST encode four elements: WHERE, WHY, HOW, and EXPECTED RESULT.
+
+Format: "[WHERE] [HOW] to [WHY] — expect [RESULT]"
+
+GOOD:
+- "src/utils/validation.ts: Add validateEmail() for input sanitization — returns boolean"
+- "UserService.create(): Call validateEmail() before DB insert — rejects invalid emails with 400"
+- "validation.test.ts: Add test for missing @ sign — expect validateEmail('foo') to return false"
+
+BAD:
+- "Implement email validation" (where? how? what result?)
+- "Add dark mode" (this is a feature, not a todo)
+- "Fix auth" (what file? what changes? what's expected?)
+
+## Granularity Rules
+
+Each todo MUST be a single atomic action completable in 1-3 tool calls. If it needs more, split it.
+
+**Size test**: Can you complete this todo by editing one file or running one command? If not, it's too big.
+
+## Task Management
+- One in_progress at a time. Complete it before starting the next.
+- Mark completed immediately after finishing each item.
+- Skip this tool for single trivial tasks (one-step, obvious action).`

src/hooks/todo-description-override/hook.ts

@@ -0,0 +1,14 @@
+import { TODOWRITE_DESCRIPTION } from "./description"
+
+export function createTodoDescriptionOverrideHook() {
+  return {
+    "tool.definition": async (
+      input: { toolID: string },
+      output: { description: string; parameters: unknown },
+    ) => {
+      if (input.toolID === "todowrite") {
+        output.description = TODOWRITE_DESCRIPTION
+      }
+    },
+  }
+}

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")
+      })
+    })
+  })
+})

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

@@ -0,0 +1 @@
+export { createTodoDescriptionOverrideHook } from "./hook"