docs/nixpkgs: Expand upgrade workflow

Move the skill to the shared agent location while retaining Claude
compatibility. Document kernel refreshes and the release metadata
policy as part of each upgrade, and make commit-message wrapping
explicit and verifiable.

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

@@ -0,0 +1,27 @@
+---
+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, update kernel and
+  release metadata, refresh pins, sweep version-gated TODOs, and review other inputs. Use when the
+  user wants to update/bump nixpkgs, refresh the pins, or do the periodic nixpkgs/home-manager
+  upgrade.
+---
+
+# Upgrade nixpkgs
+
+Read [`docs/nixpkgs-upgrade.md`](../../../docs/nixpkgs-upgrade.md), the canonical procedure, and
+follow its phases in order.
+
+Key reminders (see the doc for the full steps):
+
+- It is **guided, not automated** — do the mechanical and investigative work but stop at the ⏸
+  points: pushing the fork, resolving rebase conflicts, applying a stable-channel bump, choosing a
+  new release codename, and deleting version guards. Report the findings 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.
+- 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.