opencode: handle string store paths in skill sources

Add support for store directory paths provided as strings, in addition
to the existing path type check for directories.
Context: certain project will include a `SKILL.md` file, and instead of
managing them via some 3rd party manager or manually, why not include
them from the source itself! I implemented this in my setup, since I
wanted to include the `SKILL.md` file from the
[beads](https://github.com/steveyegge/beads) project.

modules/programs/opencode.nix

@@ -230,7 +230,13 @@ in
     };
 
     skills = lib.mkOption {
-      type = lib.types.either (lib.types.attrsOf (lib.types.either lib.types.lines lib.types.path)) lib.types.path;
+      type = lib.types.either (lib.types.attrsOf (
+        lib.types.oneOf [
+          lib.types.lines
+          lib.types.path
+          lib.types.str
+        ]
+      )) lib.types.path;
       default = { };
       description = ''
         Custom agent skills for opencode.
@@ -245,6 +251,9 @@ in
         - A path to a file (creates `opencode/skill/<name>/SKILL.md`)
         - A path to a directory (creates `opencode/skill/<name>/` with all files)
 
+        This also accepts Nix store paths (e.g., source from a package), allowing you to
+        reference subdirectories within a package source.
+
         If a path is used, it is expected to contain one folder per skill name, each
         containing a {file}`SKILL.md`. The directory is symlinked to
         {file}`$XDG_CONFIG_HOME/opencode/skill/`.
@@ -268,6 +277,9 @@ in
 
           # A skill can also be a directory containing SKILL.md and other files.
           data-analysis = ./skills/data-analysis;
+
+          # A skill can also be a subdirectory within a package source (store path)
+          beads = "''${pkgs.beads.src}/claude-plugin/skills/beads";
         }
       '';
     };
@@ -443,7 +455,11 @@ in
     )
     // lib.mapAttrs' (
       name: content:
-      if lib.isPath content && lib.pathIsDirectory content then
+      if
+        (lib.isPath content && lib.pathIsDirectory content)
+        || (builtins.isString content && lib.hasPrefix builtins.storeDir content)
+
+      then
         lib.nameValuePair "opencode/skill/${name}" {
           source = content;
           recursive = true;