[docs] Formalize cluster-PR eligibility criteria in batch-and-cluster.md (verify follow-up from #49+#53)

[kiki830621]

Problem

From verification of #49 + #53 cluster (PR #58):
「Cluster-PR relatedness criterion 弱:#49 (docs) + #53 (jq fix) 是 unrelated #47 follow-ups,non-overlap file/skill。建議在 references/batch-and-cluster.md formalize "cluster-PR eligibility = same-file OR same-skill OR same-root-issue (chain only)"」
— Source: team:devils-advocate (MEDIUM)

PR #58 用 cluster-PR 把 #49 (usecase-routing.md docs) 和 #53 (idd-all-chain/SKILL.md jq filter) 共用 1 PR。但兩 issue:

• 不同 file
• 不同 skill (idd-all-chain vs reference doc)
• 唯一共通: 都是 feature: /idd-all-chain Phase 0 pre-flight check for root issue diagnosis-readiness #47 follow-up label

DA 反論:reviewer 在 1 PR 看 2 個改動,default mental model 「他們相關」,找不到關聯就 cognitive cost ↑。對比 2 個 atomic PR 可能總成本更低。

但 split 太多會 explode review queue。需要 formalize criterion。

Type

refactor (convention / discipline)

Priority

P3 (process improvement)

Strategy

在 plugins/issue-driven-dev/references/batch-and-cluster.md 加一段 「cluster-PR eligibility criteria」:

## Cluster-PR eligibility (when to bundle vs split)

Cluster-PR mode (`/idd-implement #N #M --pr`) 應在以下情境使用:

| Criterion | Meets? | Example |
|-----------|--------|---------|
| Same-file | ✓ | #34, #36 都改 `src/cluster.ts` |
| Same-skill | ✓ | 都動 `idd-implement` 各 step |
| Same-root-issue (chain mode only) | ✓ | `/idd-all-chain #N` 衍生 spawn |
| Same parent label (e.g. #47 follow-up) | ✗ | 通常太弱 — 改用 atomic PR |
| Same PR review timing | ✗ | 共用 PR 純省力,不省 review |

「Same parent label 但 file/skill 不重疊」屬 borderline — 若 review surface < 50 lines 可接受,> 50 lines 拆 atomic。

Acceptance

• batch-and-cluster.md 補 eligibility section
• Examples 涵蓋 hot vs cold cluster patterns
• Cross-link 從 idd-implement / idd-all-chain skill 指過來

Related

• cluster: chain follow-ups (#49 docs cross-link + #53 jq regex anchor) #58 (PR that motivated the criterion question)
• docs: clarify #37 close reason — supersession by #44 chain-solve? (sister from #48 diagnosis) #49, fix: idd-all-chain Step 0.4 jq filter use anchored regex (avoid false-positive on quoted text) #53 (cluster instance)

Source: surfaced during /idd-verify --pr 58 (cluster #49 #53), Step 5b triage. Master report: #58 (comment)