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/discover.nix

@@ -0,0 +1,42 @@
+# Recursive skill discovery: one `{ name; src; }` per skill in a source tree.
+lib: root:
+let
+  walk =
+    dir:
+    let
+      subdirs = lib.filterAttrs (_: type: type == "directory") (builtins.readDir dir);
+    in
+    lib.concatLists (
+      lib.mapAttrsToList (
+        entryName: _:
+        let
+          child = dir + "/${entryName}";
+        in
+        # A directory with a SKILL.md is a skill, and its subdirectories are
+        # that skill's assets rather than nested skills, so the walk stops here.
+        # A directory without one is a cosmetic container to recurse through.
+        if (builtins.readDir child) ? "SKILL.md" then
+          [
+            {
+              name = entryName;
+              src = child;
+            }
+          ]
+        else
+          walk child
+      ) subdirs
+    );
+
+  found = walk root;
+  names = map (s: s.name) found;
+  duplicates = lib.unique (lib.filter (n: lib.count (x: x == n) names > 1) names);
+in
+if duplicates != [ ] then
+  throw ''
+    Skill name collision: ${lib.concatStringsSep ", " duplicates}.
+    Two or more skills under ${toString root} resolve to the same name.
+    A skill's name is its leaf directory name and must be globally unique
+    across the whole tree, independent of the cosmetic folders above it.
+    Rename one of the colliding skills.''
+else
+  found