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/tools.go

@@ -0,0 +1,10 @@
+//go:build tools
+// +build tools
+
+// We include our tool dependencies for `go generate` here to ensure they're
+// properly tracked by the go tool. See the Go Wiki for the rationale behind this:
+// https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module.
+
+package dns
+
+import _ "golang.org/x/tools/go/packages"