Set up the Matt Pocock skills the agent flows expect

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 #793 and #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.

Read #793 and #794 first, since this setup depends on the standards they establish. Inspect the existing docs/ layout and the expected docs/agents/issue-tracker.md entry point, then review the documented /setup-matt-pocock-skills behavior. Done means the agreed labels, issue-tracker configuration, and docs layout are present and harmless for contributors without the plugin.