chore: don't use the dfinity tap to install icp-cli

[raymondk]

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[marc0olo]

@raymondk please note the frontmatter changes we did and the rule we introduced about compatibility and ## Prerequisites:

• CLI tools like icp-cli belong in compatibility frontmatter, not in Prerequisites (which is for library/package deps). most skills currently double-list it

I chatted with Claude about the install instructions:

• agents should already know how to install things, it isn't knowledge worth maintaining in a skill. it might be better to remove the install instructions entirely.

I'm also questioning if all skills really need icp-cli, e.g. anything that just calls deployed canisters from JS doesn't require it.

instead of updating the brew command (which could also be npm i -g ...) everywhere, I suggest we drop the install instructions and just declare icp-cli in compatibility only on Skills where it is actually needed.