Rename directories to plural form: skill/ → skills/, agent/ → agents/, command/ → commands/

- Rename skill/ to skills/ for consistency with naming conventions
- Rename agent/ to agents/ and command/ to commands/
- Update AGENTS.md with all directory references
- Update scripts/test-skill.sh paths
- Update prompts/athena.txt documentation

This aligns with best practices of using plural directory names and updates
all documentation to reflect the new structure.

skills/pdf/scripts/check_fillable_fields.py

@@ -0,0 +1,12 @@
+import sys
+from pypdf import PdfReader
+
+
+# Script for the Coding Agent to run to determine whether a PDF has fillable form fields. See forms.md.
+
+
+reader = PdfReader(sys.argv[1])
+if (reader.get_fields()):
+    print("This PDF has fillable form fields")
+else:
+    print("This PDF does not have fillable form fields; you will need to visually determine where to enter data")