Set up the Matt Pocock skills the agent flows expect

[purinkle]

We use Matt Pocock's skills plugin when working on this repo with Claude Code,
but the repo carries none of the setup those skills expect. Reviewing #789 hit
this straight away: /code-review looks for docs/agents/issue-tracker.md to
learn how to fetch an issue, the file is absent, and the skill told me to run
/setup-matt-pocock-skills first.

/setup-matt-pocock-skills is the documented precondition for the whole set.
It configures three things:

• the issue tracker the skills read from, which can be GitHub Issues,
• the triage labels. This repo has no labels at all today,
• the doc layout under docs/ that the skills expect.

What adopting it would buy:

• /triage gives a route for reports that arrive from other people. That is
  most of our open issues, and the oldest is from 2019.
• /code-review reviews a branch on two axes, standards and spec. It needs
  written standards to work well, which is why Add a CONTRIBUTING.md #793 and Add a CODING_STANDARDS.md for shell scripts #794 should land
  first.

Two things to settle before doing this:

• Does this belong in a public repo that 8,000 people clone? Contributors do
  not have the plugin, so the config has to be harmless to anyone without it.
• Labels are visible to everyone who files an issue. Agree the set first.

Do #793 and #794 first. Both skills read what we write there, so setting
this up before we have written anything gives the skills nothing to work
from.