Stabilize the node agent runtime loop.

Keep the NAS-side runtime bounded to the configured export path,
make WebDAV and registration behavior env-driven, and add runtime
coverage so the first storage loop can be verified locally.

Generated with [Devin](https://cli.devin.ai/docs)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/node-agent/internal/nodeagent/capacity_supported.go

@@ -0,0 +1,23 @@
+//go:build android || darwin || dragonfly || freebsd || illumos || ios || linux || netbsd || openbsd || solaris
+
+package nodeagent
+
+import (
+	"math"
+	"syscall"
+)
+
+func detectCapacityBytes(path string) *int64 {
+	var stats syscall.Statfs_t
+	if err := syscall.Statfs(path, &stats); err != nil {
+		return nil
+	}
+
+	capacity := uint64(stats.Blocks) * uint64(stats.Bsize)
+	if capacity > math.MaxInt64 {
+		return nil
+	}
+
+	value := int64(capacity)
+	return &value
+}