chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/mock/path.go

@@ -3,7 +3,6 @@ package mock
 
 import (
 	"fmt"
-	"path"
 	"path/filepath"
 	"runtime"
 )
@@ -16,7 +15,7 @@ func Path(filename string) string {
 
 	// compute the mock path
 	file, err := filepath.Abs(
-		path.Join(
+		filepath.Join(
 			filepath.Dir(thisfile),
 			"../../mocks",
 			filename,