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/syscall_aix.go

@@ -360,7 +360,7 @@ func Wait4(pid int, wstatus *WaitStatus, options int, rusage *Rusage) (wpid int,
 	var status _C_int
 	var r Pid_t
 	err = ERESTART
-	// AIX wait4 may return with ERESTART errno, while the processus is still
+	// AIX wait4 may return with ERESTART errno, while the process is still
 	// active.
 	for err == ERESTART {
 		r, err = wait4(Pid_t(pid), &status, options, rusage)