fix: use Path.name for cross-platform directory extraction

Fixes #51

The validation logic used `path.rsplit("/", 1)[-1]` which only splits on
forward slashes. On Windows, paths use backslashes, causing the full
path to be compared against frontmatter.name instead of just the
directory name.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: gotalab

src/skillport/modules/skills/internal/validation.py

@@ -62,7 +62,7 @@ def validate_skill_record(
     description = skill.get("description", "")
     lines = skill.get("lines", 0)
     path = skill.get("path", "")
-    dir_name = path.rsplit("/", 1)[-1] if path else ""
+    dir_name = Path(path).name if path else ""
 
     # A1/A2: Key existence checks (only when meta is provided)
     if meta is not None: