fix: rewrite share.md as a proper command template with YAML frontmatter

[xiaolai]

Automated audit: This PR was generated by NLPM, a natural language programming linter, running via claude-code-action. Please evaluate the diff on its merits.

What and why

commands/share.md is the only command in the plugin that lacks YAML frontmatter entirely. Instead of the frontmatter + prompt-body format that every other command uses, it is written as a README-style markdown document with headings, code fences, and prose sections.

Without a frontmatter block, the command has no name or description fields — it will not register correctly as a slash command and diverges from the plugin's own conventions.

Fix

Rewrote the file as a minimal command template matching the format of the other 7 commands:

---
name: share
description: Share a visual explainer HTML page via Vercel and return a live URL
---
<prompt body>

The prompt body preserves all functional information from the original:

• The script invocation path (bash {{skill_dir}}/scripts/share.sh $1)
• The argument ($1 = HTML file path)
• A fallback when no argument is provided (list files in ~/.agent/diagrams/ and ask the user to select)
• The expected output description (live URL + claim URL)

The README-style documentation content (usage examples, requirements section, JSON output format) was removed from the command template, as that kind of reference material belongs in SKILL.md or a separate README rather than in the executable command body.