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

skills/custom-skill-prompt/skill.toml

@@ -1,9 +1,10 @@
 ## Custom Prompt Skill Example
 ##
-## This skill uses pure prompt engineering — no code required.
-## It generates a meeting agenda from a topic and duration.
+## The prompt body and metadata (name, description) live in SKILL.md.
+## This skill.toml only declares the runtime, input schema, and optional
+## metadata that don't fit in Markdown frontmatter.
 ##
-## To test: librefang skill test ./examples/custom-skill-prompt \
+## To test: librefang skill test ./skills/custom-skill-prompt \
 ##   --input '{"topic": "Q1 planning", "duration_minutes": "30"}'
 ##
 ## For the full reference, see: docs/skill-development.md
@@ -21,18 +22,3 @@ type = "promptonly"
 [input]
 topic = { type = "string", description = "The meeting topic", required = true }
 duration_minutes = { type = "string", description = "Meeting duration in minutes", required = true }
-
-[prompt]
-template = """
-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
-"""