Update ringtail flake inputs, add flake-update pipeline (#240)

## Summary
- Update all ringtail NixOS flake inputs (nixpkgs, disko, home-manager) to latest
- Add `flake_update` Dagger function (`nix flake update`) alongside existing `flake_lock` (`nix flake lock`)
- Add how-to guide for managing the ringtail lockfile
- Update dagger and ringtail reference cards

## Deployment and Testing
- [x] `mise run provision-ringtail` — deployed successfully, `changed=2` (repo + rebuild)
- [x] `mise run services-check` — all services healthy
- [x] Doc link and index checks pass

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/240

docs/how-to/how-to.md

@@ -1,6 +1,6 @@
 ---
 title: How-To
-modified: 2026-02-17
+modified: 2026-02-22
 tags:
   - how-to
 ---
@@ -64,6 +64,12 @@ Migration and transition plans for upcoming infrastructure changes.
 | [[upgrade-grafana-helm-chart]] | Upgrade Grafana Helm chart from 8.8.2 to 11.x |
 | [[operationalize-reolink-camera]] | Cloud-free NVR with Frigate and ring buffer recording |
 
+## Ringtail
+
+| Guide | Description |
+|-------|-------------|
+| [[manage-lockfile]] | Update or lock NixOS flake inputs via Dagger |
+
 ## Zot
 
 Mikado chain for hardening the zot registry. Track progress with `mise run docs-mikado harden-zot-registry`.