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/affinity_linux.go

@@ -38,8 +38,15 @@ func SchedSetaffinity(pid int, set *CPUSet) error {
 
 // Zero clears the set s, so that it contains no CPUs.
 func (s *CPUSet) Zero() {
+	clear(s[:])
+}
+
+// Fill adds all possible CPU bits to the set s. On Linux, [SchedSetaffinity]
+// will silently ignore any invalid CPU bits in [CPUSet] so this is an
+// efficient way of resetting the CPU affinity of a process.
+func (s *CPUSet) Fill() {
 	for i := range s {
-		s[i] = 0
+		s[i] = ^cpuMask(0)
 	}
 }