chore(husky): switch pre-commit from yarn to pnpm

package.json already declares packageManager: pnpm@9.15.2 — the
yarn invocation in the pre-commit hook was the last yarn holdout
and broke on machines where corepack enforces the declared pnpm.
pnpm exec lint-staged is the canonical equivalent.

.husky/pre-commit

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-yarn lint-staged
+pnpm exec lint-staged
 
 # Run sync generator check for Python SDK
 ./.husky/pre-commit-sdk-py-sync-check.sh