feat(nixos): migrate system configuration

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

@@ -0,0 +1,37 @@
+---
+name: remove-skills
+description: Remove one or more previously added library skills from the current project.
+disable-model-invocation: true
+---
+
+Removes a skill that [`setup-skills`](../setup-skills/SKILL.md) previously
+copied into the current project, deleting both its files and its entry in
+`.claude/skills-lock.yaml` (see [LOCKFILE.md](../setup-skills/LOCKFILE.md)
+for its schema).
+
+## Steps
+
+1. Read `.claude/skills-lock.yaml`. If it doesn't exist or is empty, tell
+   the user there's nothing installed to remove and stop.
+
+2. Determine which skill(s) to remove:
+   - If the user's invocation already named a specific skill, use that —
+     if it isn't in the lockfile, say so and stop.
+   - Otherwise, list every skill currently in the lockfile and ask the
+     user to pick one (or more).
+
+3. For each skill to remove, compute its current hash
+   (`~/.claude/skills/setup-skills/hash-dir.sh .claude/skills/<name>`)
+   and compare it to the hash stored in the lockfile:
+   - If it matches (never modified since it was installed), delete
+     `.claude/skills/<name>/` and remove its lockfile entry immediately —
+     no confirmation needed, since nothing of the user's is being lost.
+   - If it differs (locally customized), tell the user it has local
+     changes that will be permanently lost and ask for confirmation
+     before deleting. If they decline, leave that skill installed and
+     move on to the next.
+
+4. Finish with a summary of what was removed and what was left in place.
+
+Done when every skill to remove has been either deleted (with its lockfile
+entry removed) or explicitly left in place with a stated reason.