feat: implement nixos-exporter

Prometheus exporter for NixOS-specific metrics including:
- Generation collector: count, current, booted, age, config mismatch
- Flake collector: input age, input info, revision behind

Includes NixOS module, flake packaging, and documentation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

go.mod

@@ -0,0 +1,17 @@
+module git.t-juice.club/torjus/nixos-exporter
+
+go 1.23
+
+require github.com/prometheus/client_golang v1.20.5
+
+require (
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/klauspost/compress v1.17.9 // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
+	github.com/prometheus/client_model v0.6.1 // indirect
+	github.com/prometheus/common v0.55.0 // indirect
+	github.com/prometheus/procfs v0.15.1 // indirect
+	golang.org/x/sys v0.22.0 // indirect
+	google.golang.org/protobuf v1.34.2 // indirect
+)