Pull agent skills from pinned flake inputs

Pull agent skills from pinned flake inputs Skills from other repos were installed by hand into ~/.claude/skills, where nothing recorded their source or version. Two had already rotted into dangling links. nix/skills.nix now lists skills by flake input, and nix/modules/skills.nix links each one into ~/.claude/skills and ~/.codex/skills. Every repo is a non-flake input named skills-, pinned in flake.lock and moved with `nix flake update skills-`. An entry names the skills to take, or omits the list to take every directory with a SKILL.md. A misspelt name fails eval instead of producing a dangling link. These are store paths, not out-of-store links like the rest of home.file: nothing should write to a third-party skill, and a read-only copy is what holds it at the locked revision. Starts with: - herdr from herdrdev/herdr, pinned to v0.9.1 to match the installed binary; main documents flags that have not shipped yet - grill-me and grilling from mattpocock/skills v1.3.1; grill-me only invokes grilling - retro, moved out of home/.config/ai/skills into skippednote/skills, so my own skills install the same way as everyone else's