Update cx skill: setup section, restructuring, clarifications

[norswap]

This PR updates the cx skill:

• The original was imho needlessly terse, which I found confusing, and I think
  the AI did too. Better to add more tokens to the skill than to let AI make
  incorrect invocations.
• Added a section on setup, so that the skill can be productively used by people
  who don't have cx installed yet or haven't installed the grammars (also
  covers adding new language to a repo, and failures specific to Claude's
  sandboxed mode which prevents fetching the grammars) (1)
• Restructured around a modified version of the help output.
• Added a few clarifications here and there, see commits msgs for details.

(1) This could be fixed by honoring the HTTP(S)_PROXY env var in the implem
btw.

Can easily squash all of this into a single commit.