nixpkgs: Refresh channels and inputs

Rebase the fork branches and refresh nixpkgs, home-manager, and the
approved ancillary inputs. Update kernel and release metadata, adapt
removed package and Home Assistant options, and keep Determinate Nix
on its tested nixpkgs revision to avoid duplicate Boost patches.

Retire Sharry and its public endpoint because copyparty replaces it.
Document the GitHub mirror gate and require real devshell and system
builds in the upgrade validation workflow.

flake.nix

@@ -35,10 +35,11 @@
     home-manager-stable.inputs.nixpkgs.follows = "nixpkgs-stable";
 
     # Determinate Nix, used as the common Nix implementation across systems, homes, the devshell and
-    # CI (see lib.my.c.nix). We build it ourselves against our pinned nixpkgs (FlakeHub's cache needs
-    # auth), so it flows through our own Harmonia cache like everything else.
+    # CI (see lib.my.c.nix). We build it ourselves (FlakeHub's cache needs auth), so it flows through
+    # our own Harmonia cache like everything else. Keep its tested nixpkgs pin: its packaging carries
+    # compatibility patches that can conflict with newer nixpkgs patches.
     determinate-nix.url = "https://flakehub.com/f/DeterminateSystems/nix-src/*";
-    determinate-nix.inputs.nixpkgs.follows = "nixpkgs-unstable";
+    determinate-nix.inputs.nixpkgs.url = "github:NixOS/nixpkgs/4382ed2b7a6839d4280a9b386db49cbc5907414d";
 
     # Stuff used by the flake for build / deployment
     # ragenix.url = "github:yaxitech/ragenix";
@@ -68,8 +69,6 @@
     openwrt-feeds.inputs.openwrt-imagebuilder.follows = "openwrt-imagebuilder";
 
     # Packages not in nixpkgs
-    sharry.url = "github:eikek/sharry";
-    sharry.inputs.nixpkgs.follows = "nixpkgs-unstable";
     borgthin.url = "github:devplayer0/borg";
     # TODO: Update borgthin so this works
     # borgthin.inputs.nixpkgs.follows = "nixpkgs-mine";