D5: missing-readme must negative-cache 404 so backlog advances (#172)

[xenodeve]

Parent

Part of #172

What to build

When POST /github/refresh/missing-readme syncs a repo whose GitHub README is 404, persist a negative-cache README snapshot so the candidate leaves the missing-readme queue. Today the controller reports synced:1 while no repo:owner/repo:readme row is written, so the same repo (e.g. narze) is selected forever and taxonomy/case-study stay blocked on eligible that can never generate.

Generators must treat the negative-cache marker as no README (skip LLM), not as empty content to expand.

Acceptance criteria

• 404 README writes a durable snapshot key that listExistingReadmeKeys sees
• Subsequent missing-readme runs do not re-select that owner/repo
• Case-study/taxonomy skip negative-cache / empty README (0 LLM)
• Unit tests cover 404 path + controller candidate decrease
• Response distinguishes synced-with-readme vs skipped-no-readme

Blocked by

None - can start immediately

[xenodeve]

EN

Fixed locally on feat/172-visitor-ready-enrichment: 404 README writes sha:missing sentinel; failed detail sync calls markReadmeAbsent; generators use isUsableReadmeSnapshot. Missing-readme candidates 26→0 in one drain.

TH

แก้บน branch แล้ว — คิว missing-readme เคลียร์ 26→0

[xenodeve]

EN

Fixed locally on feat/172-visitor-ready-enrichment: 404 README writes sha:missing sentinel; failed detail sync calls markReadmeAbsent; generators use isUsableReadmeSnapshot. Missing-readme candidates 26→0 in one drain.

TH

แก้บน branch แล้ว — คิว missing-readme เคลียร์ 26→0