fix: vendor deps and fix CI go-version, timeout, GOTOOLCHAIN (HAN-414)

- Run go mod vendor to eliminate proxy.golang.org network dependency
- Set GOFLAGS=-mod=vendor at job env level so all go commands use vendor/
- Fix go-version from 1.23 to 1.24.0 to match go.mod
- Move GOTOOLCHAIN=local to job-level env (not per-step inline)
- Set cache: false on setup-go (cache is irrelevant with vendoring)
- Add timeout-minutes: 15 so failures are caught quickly instead of 3h

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

vendor/github.com/miekg/dns/COPYRIGHT

@@ -0,0 +1,9 @@
+Copyright 2009 The Go Authors. All rights reserved. Use of this source code
+is governed by a BSD-style license that can be found in the LICENSE file.
+Extensions of the original work are copyright (c) 2011 Miek Gieben
+
+Copyright 2011 Miek Gieben. All rights reserved. Use of this source code is
+governed by a BSD-style license that can be found in the LICENSE file.
+
+Copyright 2014 CloudFlare. All rights reserved. Use of this source code is
+governed by a BSD-style license that can be found in the LICENSE file.