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/sys/unix/fdset.go

@@ -23,7 +23,5 @@ func (fds *FdSet) IsSet(fd int) bool {
 
 // Zero clears the set fds.
 func (fds *FdSet) Zero() {
-	for i := range fds.Bits {
-		fds.Bits[i] = 0
-	}
+	clear(fds.Bits[:])
 }