docs(claude-code): teach the skill-management skills the Nix layout

~/.claude/skills is generated by home-manager: the directories are real
but every leaf file is a read-only symlink into the store. The skills
that author and install skills assumed it was an ordinary writable tree.

- craft-skill: personal skills are authored in modules/claude-code/skills
  and applied by a rebuild, never edited under ~/.claude/skills; writing
  there succeeds silently and strands the skill outside the repo.
- setup-skills, update-skills: copy out of the library with cp -rL and
  chmod -R u+w. A plain cp -r copies the symlinks, committing store paths
  into the project, and dereferenced files keep the store's read-only mode.
- craft-skill also staged through `dot add`, a fish function this repo no
  longer carries; plain git add replaces it.

modules/claude-code/skills/update-skills/SKILL.md

@@ -36,8 +36,11 @@ skill that isn't already there — that's
    already included an explicit go-ahead argument (e.g. `-y`, `yes`), in
    which case apply them without asking. Applying means: delete
    `.claude/skills/<name>/` entirely and copy
-   `~/.claude/skills/library/<name>/` in its place, so no file the project
-   copy had but the library no longer has can survive — then recompute its
+   `~/.claude/skills/library/<name>/` in its place with `cp -rL` followed
+   by `chmod -R u+w` (the library is read-only symlinks into the Nix store;
+   a plain `cp -r` would put store paths into the project), so no file the
+   project copy had but the library no longer has can survive — then
+   recompute its
    hash and overwrite that entry's `hash` in `.claude/skills-lock.yaml` in
    place.