fix: anchor .sisyphus path check to ctx.directory to prevent false positives

- Uses path.join(ctx.directory, '.sisyphus') + sep as prefix instead of loose .includes()
- Prevents false positive when .sisyphus exists in parent directories outside project root
- Adds test for the false positive case (cubic review feedback)

src/hooks/write-existing-file-guard/index.ts

@@ -1,6 +1,6 @@
 import type { Hooks, PluginInput } from "@opencode-ai/plugin"
 import { existsSync } from "fs"
-import { resolve, isAbsolute } from "path"
+import { resolve, isAbsolute, join, sep } from "path"
 import { log } from "../../shared"
 
 export function createWriteExistingFileGuardHook(ctx: PluginInput): Hooks {
@@ -20,7 +20,8 @@ export function createWriteExistingFileGuardHook(ctx: PluginInput): Hooks {
       const resolvedPath = isAbsolute(filePath) ? filePath : resolve(ctx.directory, filePath)
 
       if (existsSync(resolvedPath)) {
-        const isSisyphusMarkdown = resolvedPath.replace(/\\/g, "/").includes("/.sisyphus/") && filePath.endsWith(".md")
+        const sisyphusRoot = join(ctx.directory, ".sisyphus") + sep
+        const isSisyphusMarkdown = resolvedPath.startsWith(sisyphusRoot) && resolvedPath.endsWith(".md")
         if (isSisyphusMarkdown) {
           log("[write-existing-file-guard] Allowing .sisyphus/*.md overwrite", {
             sessionID: input.sessionID,