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.

.agents/skills/upgrade-nixpkgs/SKILL.md

@@ -22,6 +22,12 @@ Key reminders (see the doc for the full steps):
   and the `flake.nix` stable pins must agree on one release.
 - **Re-verify the patch stack against freshly fetched upstream**, not stale refs — enumerate it with
   `git log`; don't assume a remembered list.
+- After pushing the rebased fork branches, **wait for the GitHub mirror to catch up** before
+  refreshing flake pins. The `nixpkgs-mine*` inputs fetch from GitHub, not the fork's primary
+  remote; verify both GitHub branch tips match the pushed local tips first.
 - After refreshing the pins, update `lib/constants.nix` to the current explicit LTS and latest
   kernel package attributes, and update the `lib/default.nix` version overlay's `YY.MM` prefix to
   the current month. Change its codename only when the stable channel advances.
+- After the cheap evaluations pass, build the actual devshell and one representative NixOS system
+  (prefer the local box). `nix flake check --no-build` does not expose dependency build failures;
+  this is especially important when updating build-tool inputs such as Determinate Nix.