fix(frontmatter): keep the last character of a bare scalar that ends in a quote

[drewstone]

What breaks

unquote in src/frontmatter.ts strips a leading or a trailing quote independently:

function unquote(value: string): string {
  return value.replace(/^['"]|['"]$/g, '')
}

So a bare scalar that merely ends in a quote loses its last character.

written	read back
check: python3 -c "print(1)"	python3 -c "print(1)
title: the symbol '7'	the symbol '7

A truncated check is an unterminated shell quote. Every claim graded by re-executing it records unrunnable whatever the claim proved.

Why the writer never produced it, and who pays

formatYamlScalar already refuses the shape — stringNeedsJsonEncoding returns true for a value matching /["']$/ — so no page this package wrote was ever affected. The loss falls entirely on frontmatter written by another hand, which is the normal case for a store a product already had.

Measured on one such corpus (Discovery Lab, 3,655 shared pages): 1,378 pages lost a character, 1,376 of them in check, 2 in title, 1 in expect.

The fix

Strip a matched pair, which is what the writer's own encoder assumes. A quoted value still unwraps; an unmatched quote is data.

Proof

• The new case in src/frontmatter.test.ts fails on the previous rule (Tests 1 failed | 4 passed) and passes on this one.
• npx vitest run src/ — 24 files, 222 of 222 tests pass.

[tangletools]

✅ Auto-approved drewstone PR — 60007061

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

This approval is provisional. It rests on the audit running. If the audit cannot run — for example the CLI bridge rejects it — this approval is dismissed rather than left standing, so an unrun check never reads as a passing one.

_{tangletools · auto-approval · reason: drewstone_author · 2026-09-01T17:47:53Z}