[NixOS] Add bump-overlay-versions skill to repo

Move skill from global ~/.claude/skills to project-local .claude/skills
so it's version controlled with the dotfiles.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nixos/.claude/skills/bump-overlay-versions/SKILL.md

@@ -0,0 +1,80 @@
+---
+name: bump-overlay-versions
+description: Use when user asks to bump, update, or upgrade claude-code or codex versions in overlay.nix, or when checking if newer versions are available
+---
+
+# Bump Overlay Versions
+
+## Overview
+
+Updates claude-code and/or codex to latest versions in `~/dotfiles/nixos/overlay.nix`. Nix requires correct hashes which must be discovered through failed builds.
+
+## Quick Reference
+
+| Package | Check Latest | Hash Fields |
+|---------|--------------|-------------|
+| claude-code | `npm view @anthropic-ai/claude-code version` | `hash`, `npmDepsHash` (keep empty) |
+| codex | `curl -s "https://api.github.com/repos/openai/codex/releases/latest" \| jq -r '.tag_name'` | `hash`, `cargoHash` |
+
+## Workflow
+
+### 1. Check Current vs Latest Versions
+
+```bash
+# Current versions are in overlay.nix claudeCodeVersion and codexVersion blocks
+# Check latest:
+npm view @anthropic-ai/claude-code version
+curl -s "https://api.github.com/repos/openai/codex/releases/latest" | jq -r '.tag_name'
+# codex tag format: rust-v0.92.0 -> version is 0.92.0
+```
+
+### 2. Update Version and Clear Hashes
+
+In `~/dotfiles/nixos/overlay.nix`:
+
+**For claude-code:**
+```nix
+claudeCodeVersion = {
+  version = "X.Y.Z";        # Update to new version
+  hash = "";                # Clear - will get from build error
+  npmDepsHash = "";         # Keep empty (no npm deps)
+};
+```
+
+**For codex:**
+```nix
+codexVersion = {
+  version = "X.Y.Z";        # Update to new version (without rust-v prefix)
+  hash = "";                # Clear - will get from build error
+  cargoHash = "";           # Clear - will get from build error
+};
+```
+
+### 3. Build to Get Hashes
+
+```bash
+just switch
+```
+
+The build will fail with hash mismatch errors showing the correct hash. Copy the `got:` hash value.
+
+**For codex:** You'll need to run twice - once for `hash` (source), once for `cargoHash` (cargo deps).
+
+### 4. Update with Correct Hashes
+
+Replace empty strings with the hashes from build errors, then run `just switch` again.
+
+## Enable/Disable Overrides
+
+The overlay uses boolean flags - set to `false` to use nixpkgs versions instead:
+
+```nix
+enableCodexOverride = true;       # Set false to use nixpkgs codex
+enableClaudeCodeOverride = true;  # Set false to use nixpkgs claude-code
+```
+
+**Never delete the override code** - just toggle the flags.
+
+## File Location
+
+`~/dotfiles/nixos/overlay.nix`