a ratchet on hardcoded colour, and the docs to match

The three previous attempts at this job died of endlessness, not difficulty. Nothing
stopped the next component arriving with its own `text-emerald-400` while an old one
was being fixed, so the number never fell for long.

`bun run check:theme` counts palette classes per directory against a committed
baseline. It does not care what the number is, only which way it moves. 942 across 21
directories today. Every directory can be paid down independently and none can quietly
regrow — which turns an infinite job into a finite one.

A baseline rather than a ban, deliberately: a rule forbidding palette classes outright
fails on its first run, and a check nobody can make pass is a check everybody turns
off.

Proven by adding one `text-emerald-400` and watching it fail with the directory, the
delta, and what to use instead.

── Documentation, which is part of the work rather than after it ──

`src/workspaces/theme/README.md` is new and is the contract: how to write a theme, all
84 declarations by family, why colours are unwrapped HSL triplets while surfaces are
complete CSS values, why assets are quoted paths, and the rules for components. Start
from Plain, not from Duck — every value you change is then one you decided about.

`plugins/EXTRACTING-A-PLUGIN.md` gains a rule beside the others: a plugin never
hardcodes a colour, and pays its debt down DURING the extraction. The counts are named
there — Soulseek 313, Invoices 95, Wallet 68 — because "some colours" does not convey
that it is two thirds of the platform's total. The kickoff prompt says the same thing
in the form a fresh agent will meet it.

That is what makes the plugin exemption safe rather than a way of losing the work: the
ratchet stops covering a directory the moment it leaves, so the rule has to travel with
it.

`THEMES-NOTES.md` rewritten to match reality — it still listed done things as open.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

plugins/EXTRACT-A-PLUGIN-PROMPT.md

@@ -82,3 +82,17 @@ still open.
 Then update `plugins/EXTRACTING-A-PLUGIN.md`: add what this extraction taught, and correct anything it
 told you that turned out to be wrong. The runbook is the thing that makes the next one cheaper, and it is
 only worth that if each extraction leaves it more accurate than it found it.
+
+
+## Colour is part of the job
+
+The feature you are extracting almost certainly hardcodes Tailwind palette classes — `text-emerald-400`,
+`bg-zinc-800`, `border-red-500/30`. Replace them with semantic tokens as you go: `text-success`,
+`bg-muted`, `border-destructive`, `bg-busy/15`. They follow the theme; a palette number does not.
+
+Read `src/workspaces/theme/README.md` for the contract and `src/workspaces/components/Data/tone.ts` for the
+closed set of state tones. `bun run check:theme` counts what is left.
+
+Do it during the extraction rather than after. You are already rewriting every import in these files, and
+once the directory leaves the platform the ratchet stops covering it.
+