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>

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

@@ -0,0 +1,25 @@
+---
+name: upgrade-nixpkgs
+description: >-
+  Upgrade all four nixpkgs channels (unstable, stable, mine, mine-stable) and home-manager for this
+  flake: check for a NixOS stable bump, rebase the devplayer0 nixpkgs fork against upstream, run the
+  update commands, sweep version-gated TODOs, and review flake inputs. Use when the user wants to
+  update/bump nixpkgs, refresh the pins, or do the periodic nixpkgs/home-manager upgrade.
+---
+
+# Upgrade nixpkgs
+
+The canonical, agent-agnostic procedure lives in the repo at
+[`docs/nixpkgs-upgrade.md`](../../../docs/nixpkgs-upgrade.md). Read it and follow the phases in
+order.
+
+Key reminders (see the doc for the full steps):
+
+- It is **guided, not automated** — do the mechanical/investigative work but stop at the ⏸ points:
+  pushing the fork, resolving rebase conflicts, editing the `flake.nix` stable pins, and deleting
+  version guards. Report and let the user decide.
+- **Check the current NixOS stable first** (Phase 1) — the fork's `devplayer0-stable` rebase target
+  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 (stale `upstream/*` refs make already-upstreamed commits
+  masquerade as fork-only patches).