docs: regenerate WALKTHROUGH.md as plain markdown, and declare prettier

[philoserf]

Two coupled changes: regenerating the walkthrough without showboat is what exposed the formatting gap, so they land together.

The document

code-walkthrough no longer builds this file with showboat, so it is ordinary markdown now.

	before	after
lines	1,429	1,096
fenced blocks	94	41
captured output vs prose	1.8×	—

The old form paired every snippet with a captured output block, and 39 of its 47 commands were sed ranges whose only assertion was that some text still matched a file. Snippets are now quoted inline and labelled `file` — `symbol` rather than by line range.

Dropping showboat also removed constraints that were shaping content, not just formatting:

• What the tests cover shows real bun test output instead of a grep of test names — that was a workaround for verify breaking on non-deterministic output.
• Architecture is a table instead of a shell loop — a workaround for verify breaking on repo-tracking output.
• The export demo no longer needs a frozen timestamp.

Since nothing checks this document any more, fidelity was verified rather than assumed: every labelled ts block was diffed against the source file it names — 271 substantive lines, 0 mismatches.

Prettier

That check initially failed, which is why this PR also adds prettier.

The user-level auto-format hook runs prettier on any markdown it sees. This repo declared no prettier config, so prettier used its defaults — including embeddedLanguageFormatting: "auto", which rewrote quoted TypeScript into double quotes at width 80, against this repo's Biome style of single quotes at width 100. Three snippets stopped matching their source.

Biome 2.5 does not process markdown at all (Checked 0 files), so this is not a redundant second formatter — it is the only tool that can do this job. Prettier is now a declared devDependency wired into check, check:ci and format, scoped to **/*.md so it never fights Biome over TypeScript or JSON.

• .prettierrc.json — proseWrap: preserve, embeddedLanguageFormatting: off. The second is load-bearing, not stylistic.
• .prettierignore — excludes .issues/, which is ignored through the global core.excludesfile rather than this repo's .gitignore, so git hides it and prettier does not. Without this, check:ci fails on agent working notes that never ship.

Markdown drift now fails the pre-push hook like any other formatting. Verified by appending bad whitespace to README.md and confirming check:ci blocked, then restoring it.

The same gap exists across the workspace and is now tracked: 14 repos with no prettier at all, and 7 that run it with no config.

Noise in the diff

THEORY.md's 50 changed lines are prettier normalizing *emphasis* to _emphasis_ — nothing else. CLAUDE.md gains a Formatting section recording the split and why the config settings matter.

Checks

bun run check:ci green — Biome clean, prettier clean, tsc --noEmit clean, 82 tests pass. No source changes.