Expose lib.mkSkillsShellHook, which takes a list of selected skill derivations
and returns a dev-shell shellHook string. On shell entry the hook symlinks each
selected skill as a direct child of the project's .claude/skills/, pointing into
the store. It is stateless: each entry first removes only store-pointing symlinks
(deselected skills included), then relinks the current selection, leaving
hand-authored real directories untouched and regenerating a self-ignoring
.gitignore of the managed names. Add a nix flake check that sources the produced
hook against a fixture project and asserts placement, stale-removal, the
collision guard, non-store-symlink survival, and the .gitignore contents.
Add the content tier: a standalone flake that auto-discovers each skill
(a directory containing a SKILL.md, at any depth) and exposes it as an
individually addressable derivation built by lib.mkSkill. Directories
without a SKILL.md are descended through as cosmetic containers; once a
SKILL.md is found, that directory's subfolders are its assets, not
further skills. Skill names must be globally unique across the tree — a
collision is a hard eval-time error, not a warning.
A fixture-driven skill-build check under `nix flake check` exercises the
recursive walk, the builder, the SKILL.md-at-$out-root contract, the
eval-time name passthru, and the collision error. The repo ships no real
skill content yet, so packages.<system> is empty today.
