chore: build system skill body (#42)

* feat: extract SKILL.md body into AGENTS.md with H1 title and Overview section

Build system now parses SKILL.md body to extract H1 heading as the AGENTS.md
title and places remaining content under an Overview section. Adds validation
that SKILL.md body starts with H1, directory name is kebab-case, and name
field matches directory name.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: AGENTS.md is now SKILL.md body with frontmatter stripped

Build now generates AGENTS.md by extracting the SKILL.md markdown body
(everything after YAML frontmatter). CLAUDE.md remains a symlink to
AGENTS.md. Removes content generation logic (Structure, Usage, Overview,
Reference Categories, Available References sections) — SKILL.md is the
single source of truth for agent instructions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: add Structure and Usage sections to AGENTS.md, validate H1 title matches directory name

Build now generates AGENTS.md as: H1 Title > Structure > Usage > rest of
SKILL.md body. Validates that SKILL.md body starts with H1 heading and
that the title in kebab-case matches the directory name.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

AGENTS.md

@@ -10,7 +10,7 @@ Guidance for AI coding agents working with this repository.
 skills/
   {skill-name}/
     SKILL.md              # Required: skill manifest (Agent Skills spec)
-    AGENTS.md             # Generated: navigation guide for agents
+    AGENTS.md             # Generated: SKILL.md body (frontmatter stripped)
     CLAUDE.md             # Generated: symlink to AGENTS.md
     references/
       _sections.md        # Required: section definitions