Add Nix flake for mcp and mcp-agent

Exposes two packages:
- default (mcp CLI) for operator workstations
- mcp-agent for managed nodes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/dustin/go-humanize/ordinals.go

@@ -0,0 +1,25 @@
+package humanize
+
+import "strconv"
+
+// Ordinal gives you the input number in a rank/ordinal format.
+//
+// Ordinal(3) -> 3rd
+func Ordinal(x int) string {
+	suffix := "th"
+	switch x % 10 {
+	case 1:
+		if x%100 != 11 {
+			suffix = "st"
+		}
+	case 2:
+		if x%100 != 12 {
+			suffix = "nd"
+		}
+	case 3:
+		if x%100 != 13 {
+			suffix = "rd"
+		}
+	}
+	return strconv.Itoa(x) + suffix
+}