build: fix pre-commit hook for symlinked self-dep and staged deletions (#621)

- link-self.mjs: skip when node_modules/@modelcontextprotocol/ext-apps is
  a symlink (e.g. to the repo root). cpSync("dist", target/dist) was
  failing with ERR_FS_CP_EINVAL because src and dest resolved to the
  same path.
- .husky/pre-commit: capture staged files with --diff-filter=d so the
  re-stage step doesn't fail with "pathspec did not match" on commits
  (typically merges) that include deletions.

scripts/link-self.mjs

@@ -5,10 +5,12 @@
  * so examples always type-check against the latest local types.
  * See: https://github.com/npm/feedback/discussions/774
  */
-import { cpSync, existsSync } from "fs";
+import { cpSync, existsSync, lstatSync } from "fs";
 
 const target = "node_modules/@modelcontextprotocol/ext-apps";
 if (!existsSync(target)) process.exit(0);
+// If target is a symlink (e.g. to the repo root), dist is already current.
+if (lstatSync(target).isSymbolicLink()) process.exit(0);
 
 cpSync("dist", `${target}/dist`, { recursive: true });
 cpSync("package.json", `${target}/package.json`);