refactor(skills): make SKILL.md the required entry point

Standardize on Claude Code's SKILL.md format as every skill's source of
truth. skill.toml becomes an optional metadata layer for runtime, input
schema, and versioning — never for the prompt body.

- validate.py: require SKILL.md in every skill dir; when skill.toml also
  exists, cross-check name/description consistency to prevent drift
- Add SKILL.md to the two custom-skill examples
- Move the meeting-agenda prompt body out of skill.toml into SKILL.md
- Rewrite skills/README.md to document the md-first, toml-as-metadata convention

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skills/custom-skill-python/SKILL.md

@@ -0,0 +1,11 @@
+---
+name: word-counter
+description: Count words, sentences, and characters in text.
+---
+
+# Word Counter
+
+Counts words, sentences, and characters in the provided text.
+
+The prompt body is unused for this skill — execution is delegated to
+`main.py` via the `python` runtime declared in `skill.toml`.