feat(write-existing-file-guard): add hook to prevent write tool from overwriting existing files

Adds a PreToolUse hook that intercepts write operations and throws an error
if the target file already exists, guiding users to use the edit tool instead.

- Throws error: 'File already exists. Use edit tool instead.'
- Hook is enabled by default, can be disabled via disabled_hooks
- Includes comprehensive test suite with BDD-style comments

src/index.ts

@@ -37,6 +37,7 @@ import {
   createUnstableAgentBabysitterHook,
   createPreemptiveCompactionHook,
   createTasksTodowriteDisablerHook,
+  createWriteExistingFileGuardHook,
 } from "./hooks";
 import {
   contextCollector,
@@ -280,6 +281,9 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
 
   const questionLabelTruncator = createQuestionLabelTruncatorHook();
   const subagentQuestionBlocker = createSubagentQuestionBlockerHook();
+  const writeExistingFileGuard = isHookEnabled("write-existing-file-guard")
+    ? createWriteExistingFileGuardHook(ctx)
+    : null;
 
   const taskResumeInfo = createTaskResumeInfoHook();
 
@@ -720,6 +724,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
 
     "tool.execute.before": async (input, output) => {
       await subagentQuestionBlocker["tool.execute.before"]?.(input, output);
+      await writeExistingFileGuard?.["tool.execute.before"]?.(input, output);
       await questionLabelTruncator["tool.execute.before"]?.(input, output);
       await claudeCodeHooks["tool.execute.before"](input, output);
       await nonInteractiveEnv?.["tool.execute.before"](input, output);