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

nixos/ringtail/flake.lock

@@ -7,11 +7,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1771355198,
-        "narHash": "sha256-89m5VKxIs8QNiIvLsxHu5NpyhDsoXTtoN801IAurnW4=",
+        "lastModified": 1771469470,
+        "narHash": "sha256-GnqdqhrguKNN3HtVfl6z+zbV9R9jhHFm3Z8nu7R6ml0=",
         "owner": "nix-community",
         "repo": "disko",
-        "rev": "92fceb111901a6f13e81199be4fab95fce86a5c9",
+        "rev": "4707eec8d1d2db5182ea06ed48c820a86a42dc13",
         "type": "github"
       },
       "original": {
@@ -27,11 +27,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1770260404,
-        "narHash": "sha256-3iVX1+7YUIt23hBx1WZsUllhbmP2EnXrV8tCRbLxHc8=",
+        "lastModified": 1771744638,
+        "narHash": "sha256-EDLi+YAsEEAmMeZe1v6GccuGRbCkpSZp/+A6g+pivR8=",
         "owner": "nix-community",
         "repo": "home-manager",
-        "rev": "0d782ee42c86b196acff08acfbf41bb7d13eed5b",
+        "rev": "cb6c151f5c9db4df0b69d06894dc8484de1f16a0",
         "type": "github"
       },
       "original": {
@@ -43,11 +43,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1771208521,
-        "narHash": "sha256-X01Q3DgSpjeBpapoGA4rzKOn25qdKxbPnxHeMLNoHTU=",
+        "lastModified": 1771574726,
+        "narHash": "sha256-D1PA3xQv/s4W3lnR9yJFSld8UOLr0a/cBWMQMXS+1Qg=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "fa56d7d6de78f5a7f997b0ea2bc6efd5868ad9e8",
+        "rev": "c217913993d6c6f6805c3b1a3bda5e639adfde6d",
         "type": "github"
       },
       "original": {