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.

.claude/skills/upgrade-nixpkgs

@@ -0,0 +1 @@
+../../.agents/skills/upgrade-nixpkgs

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

@@ -1,25 +0,0 @@
----
-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).