claude-code: fix skills implementation

Claude skills are only found when using `SKILL.md` entrypoints.
Attribute names should be used for the directory structure, not the
filename.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

modules/programs/claude-code.nix

@@ -298,14 +298,14 @@ in
       default = { };
       description = ''
         Custom skills for Claude Code.
-        The attribute name becomes the skill filename or directory name, and the value is either:
-        - Inline content as a string (creates .claude/skills/<name>.md)
-        - A path to a file (creates .claude/skills/<name>.md)
+        The attribute name becomes the skill directory name, and the value is either:
+        - Inline content as a string (creates .claude/skills/<name>/SKILL.md)
+        - A path to a file (creates .claude/skills/<name>/SKILL.md)
         - A path to a directory (creates .claude/skills/<name>/ with all files)
       '';
       example = lib.literalExpression ''
         {
-          xlsx = ./skills/xlsx.md;
+          xlsx = ./skills/xlsx/SKILL.md;
           data-analysis = ./skills/data-analysis;
           pdf-processing = '''
             ---
@@ -334,8 +334,9 @@ in
       type = lib.types.nullOr lib.types.path;
       default = null;
       description = ''
-        Path to a directory containing skill files for Claude Code.
-        Skill files from this directory will be symlinked to .claude/skills/.
+        Path to a directory containing skill directories for Claude Code.
+        Each skill directory should contain a SKILL.md entrypoint file.
+        Skill directories from this path will be symlinked to .claude/skills/.
       '';
       example = lib.literalExpression "./skills";
     };
@@ -516,7 +517,7 @@ in
             recursive = true;
           }
         else
-          lib.nameValuePair ".claude/skills/${name}.md" (
+          lib.nameValuePair ".claude/skills/${name}/SKILL.md" (
             if lib.isPath content then { source = content; } else { text = content; }
           )
       ) cfg.skills;

tests/modules/programs/claude-code/mixed-content.nix

@@ -38,14 +38,14 @@
     assertFileExists home-files/.claude/commands/path-command.md
     assertFileExists home-files/.claude/agents/inline-agent.md
     assertFileExists home-files/.claude/agents/path-agent.md
-    assertFileExists home-files/.claude/skills/inline-skill.md
-    assertFileExists home-files/.claude/skills/path-skill.md
+    assertFileExists home-files/.claude/skills/inline-skill/SKILL.md
+    assertFileExists home-files/.claude/skills/path-skill/SKILL.md
 
     assertFileContent home-files/.claude/commands/path-command.md \
       ${./test-command.md}
     assertFileContent home-files/.claude/agents/path-agent.md \
       ${./test-agent.md}
-    assertFileContent home-files/.claude/skills/path-skill.md \
+    assertFileContent home-files/.claude/skills/path-skill/SKILL.md \
       ${./test-skill.md}
   '';
 }

tests/modules/programs/claude-code/skill-subdir/SKILL.md

@@ -0,0 +1,12 @@
+---
+name: data-processing
+description: Process and transform structured data files.
+---
+
+# Data Processing Skill
+
+Use this skill for data-processing tasks.
+
+## Supporting files
+- `extract.md`: extraction workflow details
+- `convert.md`: conversion workflow details

tests/modules/programs/claude-code/skills-dir.nix

@@ -5,10 +5,10 @@
   };
 
   nmt.script = ''
-    assertFileExists home-files/.claude/skills/test-skill.md
-    assertLinkExists home-files/.claude/skills/test-skill.md
+    assertFileExists home-files/.claude/skills/test-skill/SKILL.md
+    assertLinkExists home-files/.claude/skills/test-skill/SKILL.md
     assertFileContent \
-      home-files/.claude/skills/test-skill.md \
-      ${./skills/test-skill.md}
+      home-files/.claude/skills/test-skill/SKILL.md \
+      ${./skills/test-skill/SKILL.md}
   '';
 }

tests/modules/programs/claude-code/skills-path.nix

@@ -7,8 +7,8 @@
   };
 
   nmt.script = ''
-    assertFileExists home-files/.claude/skills/test-skill.md
-    assertFileContent home-files/.claude/skills/test-skill.md \
+    assertFileExists home-files/.claude/skills/test-skill/SKILL.md
+    assertFileContent home-files/.claude/skills/test-skill/SKILL.md \
       ${./test-skill.md}
   '';
 }

tests/modules/programs/claude-code/skills-subdir.nix

@@ -7,10 +7,15 @@
   };
 
   nmt.script = ''
+    assertFileExists home-files/.claude/skills/data-processing/SKILL.md
     assertFileExists home-files/.claude/skills/data-processing/extract.md
     assertFileExists home-files/.claude/skills/data-processing/convert.md
+    assertLinkExists home-files/.claude/skills/data-processing/SKILL.md
     assertLinkExists home-files/.claude/skills/data-processing/extract.md
     assertLinkExists home-files/.claude/skills/data-processing/convert.md
+    assertFileContent \
+      home-files/.claude/skills/data-processing/SKILL.md \
+      ${./skill-subdir/SKILL.md}
     assertFileContent \
       home-files/.claude/skills/data-processing/extract.md \
       ${./skill-subdir/extract.md}