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/config/schema.ts

@@ -94,6 +94,7 @@ export const HookNameSchema = z.enum([
   "unstable-agent-babysitter",
   "stop-continuation-guard",
   "tasks-todowrite-disabler",
+  "write-existing-file-guard",
 ])
 
 export const BuiltinCommandNameSchema = z.enum([