Add current Markdown-to-PDF agent skill

[0xtlt]

Summary

• add a modern render-markdown-pdf Agent Skill with Codex UI metadata
• cover current document, batch, merge, ZIP, Mermaid, syntax highlighting, and 16:9 slide-deck workflows
• document project-local and global skill installation

Validation

• cargo fmt --check
• cargo clippy --all-targets --locked -- -D warnings
• cargo test --locked — 55 unit tests and 25 CLI integration tests passed
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps --locked
• quick_validate.py .agents/skills/render-markdown-pdf
• npx --yes skills add ./ --list — discovered exactly render-markdown-pdf
• forward-tested the slide workflow, including preservation of md2pdf overflow warnings

Context

Supersedes closed PR #11, which predated batch conversion and slide-deck support.

No demo video: this change adds agent instructions and README documentation; it does not change rendered output or frontend visuals.