feat(nixos): migrate system configuration

configs/nixos/modules/agents/claude-code/skills/setup-skills/SKILL.md

@@ -0,0 +1,53 @@
+---
+name: setup-skills
+description: Add relevant skills from the shared skills library to the current project.
+disable-model-invocation: true
+---
+
+Adds opt-in, project-specific skills from `~/.claude/skills/library/` into
+the current project's `.claude/skills/`, tracked in
+`.claude/skills-lock.yaml` (see [LOCKFILE.md](LOCKFILE.md) for its schema).
+Only ever adds — checking already-installed skills for updates is
+[`update-skills`](../update-skills/SKILL.md)'s job, not this one's.
+
+The library is a tree of read-only symlinks into the Nix store, so every
+copy out of it must dereference (`cp -rL`) and then restore write
+permission (`chmod -R u+w`). A plain `cp -r` copies the symlinks
+themselves, putting store paths into the project that break on any other
+machine.
+
+## Steps
+
+1. Read `.claude/skills-lock.yaml` in the current project, if it exists.
+   Note every skill name already listed — these are already installed and
+   must not be re-proposed.
+
+2. List every skill under `~/.claude/skills/library/*/SKILL.md` and read
+   each one's `name` and `description`.
+
+3. Inspect the current project (file tree, manifests like
+   `pyproject.toml`/`package.json`, file extensions present, etc.) and
+   judge which library skills — excluding ones already installed — seem
+   relevant, the same way you'd reason about any unfamiliar codebase.
+   Propose that shortlist to the user with your reasoning, one line per
+   skill. If the user asks to see the full catalog instead, list every
+   library skill (minus already-installed ones) with its description.
+
+4. Let the user confirm, adjust, or pick freely from the full list.
+
+5. For each confirmed skill:
+   - If `.claude/skills/<name>/` already exists in the project and is
+     *not* in the lockfile, skip it and tell the user why (a same-named
+     skill already lives there and isn't tracked — remove or rename it
+     first if they want the library version).
+   - Otherwise, copy `~/.claude/skills/library/<name>/` to
+     `.claude/skills/<name>/` in the project with
+     `cp -rL` followed by `chmod -R u+w`, run
+     `~/.claude/skills/setup-skills/hash-dir.sh .claude/skills/<name>`,
+     and append `{name, hash: <output>}` to `.claude/skills-lock.yaml`
+     (create the file, an empty YAML list, if it doesn't exist yet).
+
+6. Report what was added and what was skipped, and why.
+
+Done when every confirmed skill is either copied and recorded in the
+lockfile, or explicitly skipped with a stated reason.