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-prompt/SKILL.md

@@ -0,0 +1,16 @@
+---
+name: meeting-agenda
+description: Generate a structured meeting agenda from a topic and duration.
+---
+
+Create a structured meeting agenda for the following:
+
+Topic: {{topic}}
+Duration: {{duration_minutes}} minutes
+
+Requirements:
+- Include time allocations for each section
+- Start with a brief intro/alignment (2-3 min)
+- End with action items and next steps (3-5 min)
+- Keep sections focused and actionable
+- Format as a numbered list with time in brackets