feat: package skills as per-skill Nix derivations (task 0001)

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.

lib/mk-skill.nix

@@ -0,0 +1,27 @@
+# Builds a skill source directory into a derivation, `SKILL.md` at the `$out` root.
+{
+  pkgs,
+  src,
+  name ? builtins.baseNameOf (toString src),
+}:
+pkgs.stdenvNoCC.mkDerivation {
+  inherit name src;
+
+  # A skill is just files to copy, with nothing to unpack, configure, or build.
+  dontUnpack = true;
+  dontConfigure = true;
+  dontBuild = true;
+
+  installPhase = ''
+    runHook preInstall
+    mkdir -p "$out"
+    cp -R "$src"/. "$out/"
+    test -f "$out/SKILL.md" \
+      || { echo "mkSkill: skill '${name}' has no SKILL.md at its root" >&2; exit 1; }
+    runHook postInstall
+  '';
+
+  # The name as an eval-time attribute, readable without building `$out`
+  # (no import-from-derivation).
+  passthru.skillName = name;
+}