Document external plugin dependencies

[GoBeromsu]

Summary

Several skills reference external CLI tools and plugins that users need to install separately. These dependencies should be documented clearly so users don't hit missing-tool errors.

External dependencies

Skill	Dependency	Install
youtube	defuddle	npm install -g defuddle
youtube	yt-dlp	brew install yt-dlp
scrapling	scrapling	pip install scrapling
obsidian-sync	obsidian-headless (ob)	npm install -g obsidian-headless
obsidian-mermaid	Obsidian app	N/A (editor dependency)
terminology	obsidian CLI	separate install
youtube	obsidian CLI	separate install

Tasks

• Add a top-level DEPENDENCIES.md or a section in README listing all external tools
• Each skill's SKILL.md should note its dependencies in a ## Prerequisites section
• Consider adding a compatibility field to plugin.json if the schema supports it

Context

Users installing via claude plugins install won't see the skill's internal docs until they trigger the skill. Surfacing dependencies upfront prevents frustrating "command not found" errors.