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/hooks/index.ts

@@ -38,3 +38,4 @@ export { createCompactionContextInjector, type SummarizeContext } from "./compac
 export { createUnstableAgentBabysitterHook } from "./unstable-agent-babysitter";
 export { createPreemptiveCompactionHook } from "./preemptive-compaction";
 export { createTasksTodowriteDisablerHook } from "./tasks-todowrite-disabler";
+export { createWriteExistingFileGuardHook } from "./write-existing-file-guard";