fix(reposetup): stop flagging writer-quoted frontmatter scalars as manual review (change 0447)

[danielhanold]

↩ Change 0447 — repository check flags docket's own single-quoted frontmatter as needing manual review

docket repository check reported every correctly quoted string field (slug: 'x', title: '…', type: 'fix') as a frontmatter-manual-review error, because the checker compared the raw quoted bytes against the decoded value. That was 921 findings on the live corpus, with no way to clear them.

What changed

• New wellFormedQuotedString helper in internal/reposetup/repair.go decides on the parsed YAML node: exactly one string scalar, single- or double-quoted, no tag or anchor, and nothing after it (a second decode must hit EOF). planQuote returns no finding for such tokens.
• Bare ambiguous scalars (yes, true, unquoted : ) and malformed quoting are still reported, unchanged.
• Tests: helper table, no-finding table, still-flagged cases, a writer/checker parity table over 39 adversarial strings, and an end-to-end change.create → zero repair findings test. Each guard was mutation-checked.

Verification

• Live corpus: frontmatter-manual-review 921 → 0; other finding families unchanged (artifact-links-stale 275, drop-terminal-claimed-at 241).
• Review (docket-review-standard): no findings.

Deviation from spec: the helper also requires end-of-input, since a single YAML decode silently accepts trailing content like 'a' b. Stricter than specified.

command: go run ./cmd/docket development test
result: green
head_sha: 9b2adbb
ran_at: 2026-09-27T17:50:03Z