fix(hooks): use path.isAbsolute() for cross-platform path detection

Replace path.startsWith('/') with path.isAbsolute() in directory
injector hooks. The startsWith('/') check only works on Unix-like
systems where absolute paths begin with '/'. On Windows, absolute
paths start with drive letters (e.g., C:\), causing resolveFilePath
to incorrectly treat them as relative and prepend the project
directory.

This follows the same pattern already used in
src/features/claude-tasks/storage.ts (commit 8e349aa).

Affected hooks:
- directory-agents-injector: AGENTS.md injection
- directory-readme-injector: README.md injection

src/hooks/directory-agents-injector/finder.ts

@@ -1,11 +1,11 @@
 import { existsSync } from "node:fs";
-import { dirname, join, resolve } from "node:path";
+import { dirname, isAbsolute, join, resolve } from "node:path";
 
 import { AGENTS_FILENAME } from "./constants";
 
 export function resolveFilePath(rootDirectory: string, path: string): string | null {
   if (!path) return null;
-  if (path.startsWith("/")) return path;
+  if (isAbsolute(path)) return path;
   return resolve(rootDirectory, path);
 }
 

src/hooks/directory-readme-injector/finder.ts

@@ -1,11 +1,11 @@
 import { existsSync } from "node:fs";
-import { dirname, join, resolve } from "node:path";
+import { dirname, isAbsolute, join, resolve } from "node:path";
 
 import { README_FILENAME } from "./constants";
 
 export function resolveFilePath(rootDirectory: string, path: string): string | null {
   if (!path) return null;
-  if (path.startsWith("/")) return path;
+  if (isAbsolute(path)) return path;
   return resolve(rootDirectory, path);
 }