fix flaky web-fetch tests + lock cleanup

What:
- stub resolvePinnedHostname in web-fetch tests to avoid DNS flake
- close lock file handles via FileHandle.close during cleanup to avoid EBADF

Why:
- make CI deterministic without network/DNS dependence
- prevent double-close errors from GC

Tests:
- pnpm vitest run --config vitest.unit.config.ts src/agents/tools/web-tools.fetch.test.ts src/agents/session-write-lock.test.ts (failed: missing @aws-sdk/client-bedrock)

src/agents/session-write-lock.ts

@@ -35,8 +35,8 @@ function isAlive(pid: number): boolean {
 function releaseAllLocksSync(): void {
   for (const [sessionFile, held] of HELD_LOCKS) {
     try {
-      if (typeof held.handle.fd === "number") {
-        fsSync.closeSync(held.handle.fd);
+      if (typeof held.handle.close === "function") {
+        void held.handle.close().catch(() => {});
       }
     } catch {
       // Ignore errors during cleanup - best effort