fix: pin go directive to 1.24.0 and add GOTOOLCHAIN=local to CI

The CI was consistently failing because go.mod declared 'go 1.25.6',
which triggered Go's GOTOOLCHAIN=auto mechanism to auto-download Go
1.25.6 on the CI runner. That pre-release build lacks the 'covdata'
tool, causing 'go test -race -coverprofile=coverage.out ./...' to exit
non-zero on cmd/ packages (which have no test files), even though all
actual tests passed.

Fix:
- Lower go directive from 1.25.6 to 1.24.0 (minimum required by deps)
- Set go-version: '1.24' in setup-go to match
- Add GOTOOLCHAIN=local to every go command in ci.yml to prevent any
  further auto-download regardless of future go.mod changes

All tests pass locally with GOTOOLCHAIN=local.

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

go.mod

@@ -1,6 +1,6 @@
 module github.com/hits/ExploreDNS
 
-go 1.25.6
+go 1.24.0
 
 require (
 	github.com/miekg/dns v1.1.72