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>

vendor/golang.org/x/net/internal/socks/socks.go

@@ -297,7 +297,7 @@ func (up *UsernamePassword) Authenticate(ctx context.Context, rw io.ReadWriter,
 		b = append(b, up.Username...)
 		b = append(b, byte(len(up.Password)))
 		b = append(b, up.Password...)
-		// TODO(mikio): handle IO deadlines and cancelation if
+		// TODO(mikio): handle IO deadlines and cancellation if
 		// necessary
 		if _, err := rw.Write(b); err != nil {
 			return err