modules/profile-dev: add flake git add reminder to nix skill

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

Author: aster-void

modules/home/profile-dev/programs/claude-code/skills/nix/SKILL.md

@@ -34,6 +34,10 @@ nixos-rebuild switch      # Apply NixOS config
 home-manager switch       # Apply home-manager config
 ```
 
+## Workflow
+
+- **Always `git add` new files** - Flakes only see git-tracked files
+
 ## MCP Servers
 
 - **context7**: Use `mcp__context7__resolve-library-id` with "nix" or "nixos" to fetch docs