Initial commit: full Phases 1-6 implementation

Post-repair hardware validation pipeline for Proxmox cluster hosts.
Go orchestrator + in-image agent + mkosi live image + bundled dnsmasq
PXE + SQLite + HTMX/SSE UI + notify registry + janitor + full docs.

internal/pxe/sighup_windows.go

@@ -0,0 +1,12 @@
+//go:build windows
+
+package pxe
+
+import (
+	"fmt"
+	"os"
+)
+
+func sighup(_ *os.Process) error {
+	return fmt.Errorf("SIGHUP not supported on Windows")
+}