fix(notepad-guard): refuse Write tool for .sisyphus/notepads files (#3685)

Adds a new `notepad-write-guard` hook that intercepts Write tool calls
whose target path matches `**/.sisyphus/notepads/**` and throws an
actionable error instead of allowing the write to proceed.

Without this guard, an agent that hits an Edit hash-mismatch failure
could silently fall back to Write, destroying the entire history of an
append-only notepad file (decisions.md, issues.md, etc.).  The file
carries an explicit "NEVER overwrite" warning that the agent ignores
under context pressure.

The guard is path-based so it works regardless of plan name or nesting
depth.  Non-notepad `.sisyphus/**` paths (e.g. plan files) are
unaffected.  The hook is wired into `create-tool-guard-hooks` under the
hook name `notepad-write-guard` and follows the same safeCreateHook +
HookName schema pattern as every other tool-guard hook.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/plugin/hooks/create-tool-guard-hooks.ts

@@ -19,6 +19,7 @@ import {
   createWebFetchRedirectGuardHook,
   createTeamToolGating,
   createFsyncSkipWarningHook,
+  createNotepadWriteGuardHook,
 } from "../../hooks"
 import {
   getOpenCodeVersion,
@@ -45,6 +46,7 @@ export type ToolGuardHooks = {
   webfetchRedirectGuard: ReturnType<typeof createWebFetchRedirectGuardHook> | null
   fsyncSkipWarning: ReturnType<typeof createFsyncSkipWarningHook> | null
   teamToolGating: ReturnType<typeof createTeamToolGating> | null
+  notepadWriteGuard: ReturnType<typeof createNotepadWriteGuardHook> | null
 }
 
 export function createToolGuardHooks(args: {
@@ -145,6 +147,10 @@ export function createToolGuardHooks(args: {
     ? safeHook("fsync-skip-warning", () => createFsyncSkipWarningHook())
     : null
 
+  const notepadWriteGuard = isHookEnabled("notepad-write-guard")
+    ? safeHook("notepad-write-guard", () => createNotepadWriteGuardHook())
+    : null
+
   return {
     commentChecker,
     toolOutputTruncator,
@@ -162,5 +168,6 @@ export function createToolGuardHooks(args: {
     webfetchRedirectGuard,
     fsyncSkipWarning,
     teamToolGating,
+    notepadWriteGuard,
   }
 }