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>

CLAUDE.md

@@ -0,0 +1,90 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+nixos-exporter is a Prometheus exporter for NixOS-specific metrics. It exposes system state information that standard exporters don't cover: generation management, flake input freshness, and upgrade status.
+
+**Status**: Implementation complete.
+
+## Build Commands
+
+Run commands through the Nix development shell using `nix develop -c`:
+
+```bash
+# Build
+nix develop -c go build ./...
+
+# Run tests
+nix develop -c go test ./...
+
+# Run single test
+nix develop -c go test -run TestName ./path/to/package
+
+# Lint
+nix develop -c golangci-lint run
+
+# Vulnerability check
+nix develop -c govulncheck ./...
+
+# Test Nix build
+nix build
+
+# Run the binary (prefer this over go build + running binary)
+# To pass arguments, use -- before them: nix run .#default -- --help
+nix run .#default
+```
+
+## Testing Procedures
+
+Before committing, run the following checks:
+
+1. `nix develop -c go test ./...` - Unit tests
+2. `nix develop -c golangci-lint run` - Linting
+3. `nix develop -c govulncheck ./...` - Vulnerability scanning
+4. `nix build` - Verify nix build works
+
+## Architecture
+
+Single binary with pluggable collectors:
+
+```
+nixos-exporter/
+├── main.go                 # Entry point, HTTP server on :9971
+├── collector/
+│   ├── generation.go       # Core metrics (count, current, booted, age, mismatch)
+│   └── flake.go            # Flake input freshness metrics (optional)
+└── config/
+    └── config.go           # YAML + CLI flag handling
+```
+
+### Collectors
+
+**Generation collector** (always enabled): Reads symlinks in `/nix/var/nix/profiles/` and `/run/current-system` to determine generation state.
+
+**Flake collector** (optional): Parses `flake.lock` JSON to extract `lastModified` timestamps per input.
+
+### Configuration
+
+Supports YAML config file, CLI flags (`--listen`, `--collector.flake`, `--flake.lock-path`), and NixOS module integration via `services.prometheus.exporters.nixos`.
+
+## Commit Message Format
+
+Use conventional commit format:
+
+```
+feat: add new feature
+fix: fix a bug
+docs: update documentation
+refactor: refactor code without changing behavior
+test: add or update tests
+chore: maintenance tasks
+```
+
+## Key Design Decisions
+
+- Go chosen for mature Prometheus client library and static binary output
+- Port 9971 allocated for this exporter
+- Follows nixpkgs `services.prometheus.exporters.*` module pattern
+- No root required - only reads symlinks and files