docs: Add guided nixpkgs upgrade procedure

Capture the periodic upgrade of the four nixpkgs channels and
home-manager as a repo doc: check for a NixOS stable bump first, rebase
the devplayer0 fork against upstream (re-verifying the patch stack
against freshly fetched refs), run the update commands, sweep
version-gated TODOs, and review the remaining flake inputs.

Keep the canonical, agent-agnostic procedure in docs/ and point both
AGENTS.md and a thin Claude Code skill at it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/README.md

@@ -25,6 +25,8 @@ Not every box fits this pattern, but **colony** and **home** are organised this
 - [`networking.md`](networking.md) — network assignments, domains, site topologies, router HA,
   the AS211024 L2 mesh, BGP, WireGuard, Tailscale.
 - [`deployment.md`](deployment.md) — deploy-rs, devshell commands, secrets workflow, CI.
+- [`nixpkgs-upgrade.md`](nixpkgs-upgrade.md) — guided procedure for the periodic upgrade of the four
+  nixpkgs channels and home-manager (fork rebase, stable bumps, input review).
 - [`reference/dns.md`](reference/dns.md) — generated forward and reverse DNS record reference.
 - [`reference/nixos-options.md`](reference/nixos-options.md) — generated per-option reference for
   the custom `my.*` NixOS modules.