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.test.ts

@@ -267,6 +267,25 @@ describe("createWriteExistingFileGuardHook", () => {
         await expect(result).rejects.toThrow("File already exists. Use edit tool instead.")
       })
 
+      test("blocks write when .sisyphus is in parent path but not under ctx.directory", async () => {
+        //#given
+        const fakeSisyphusParent = path.join(os.tmpdir(), ".sisyphus", "evil-project")
+        fs.mkdirSync(fakeSisyphusParent, { recursive: true })
+        const evilFile = path.join(fakeSisyphusParent, "plan.md")
+        fs.writeFileSync(evilFile, "# Evil Plan")
+        const input = { tool: "Write", sessionID: "ses_1", callID: "call_1" }
+        const output = { args: { filePath: evilFile, content: "# Hacked" } }
+
+        //#when
+        const result = hook["tool.execute.before"]?.(input as any, output as any)
+
+        //#then
+        await expect(result).rejects.toThrow("File already exists. Use edit tool instead.")
+
+        // cleanup
+        fs.rmSync(path.join(os.tmpdir(), ".sisyphus"), { recursive: true, force: true })
+      })
+
       test("blocks write to existing regular file (not in .sisyphus)", async () => {
         //#given
         const regularFile = path.join(tempDir, "regular.md")

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,