fix: improve internal skills gitignore (#696)

Turns out skills must be in the top-level `skills` directory to be
registred by claude, so I changed the gitignore to only ignore ones
ending in `-internal.md`.

Author: szydlovsky

.gitignore

@@ -93,4 +93,4 @@ nitrogen/
 .maestro/**/*_diff.png
 
 # Claude skills
-.claude/skills/internal
+.claude/skills/*-internal.md