feat: add nixos_flake_info metric with current and remote revisions

Add a new info metric that exposes the current system's flake revision
and the latest remote revision as labels. This makes it easier to see
exactly which revision is deployed vs available.

Also adds version constant to Go code and extracts it in flake.nix,
providing a single source of truth for the version.

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

CLAUDE.md

@@ -90,6 +90,10 @@ Follow semantic versioning:
 - **Minor** (0.x.0): Non-breaking changes adding features
 - **Major** (x.0.0): Breaking changes
 
+Update the `const version` in `main.go`. The Nix build extracts the version from there automatically.
+
+**When to bump**: If any Go code has changed, bump the version before committing. Do this automatically when asked to commit. On feature branches, only bump once per branch (check if version has already been bumped compared to master).
+
 ## Key Design Decisions
 
 - Go chosen for mature Prometheus client library and static binary output