[docs] Add chain-vs-bulk note in usecase-routing.md decision tree section (verify follow-up from #49)

[kiki830621]

Problem

From verification of #49 (PR #58):
「Decision tree section (line 11-26) 未補 chain vs bulk note。Implementation Plan 提及但實作 narrow 到 row 27 + blockquote。Acceptance 已透過 row 27 達成,但與 plan letter 有微小落差」
— Source: team:requirements (49.5) + team:regression + codex

plugins/issue-driven-dev/references/usecase-routing.md 開頭 line 11-26 是 high-level decision tree(picker by skill)。#49 PR 把 chain-vs-bulk 區分放在 row 27 + blockquote,沒 patch decision tree。

實際 row 27 已涵蓋 acceptance,但若 reader 只看 decision tree 會錯過 「無內建 bulk-solve」的關鍵訊息。

Type

documentation (cross-link / discoverability)

Priority

P3 (cosmetic — main table 已 cover acceptance)

Strategy

在 line 11-26 decision tree section 末尾加 1-2 行 note,例如:

> **Note**: Decision tree only covers atomic skill picking. For backlog
> bulk-solve scenarios (N unrelated open issues), see row 27 below — there
> is no built-in zero-arg bulk-solve;use per-issue `/idd-all` 或
> `/idd-all-chain` if hot-emergent ripple expected.

Acceptance

• Decision tree section 補 1-2 line bulk-solve cross-reference
• No regression to existing decision tree readability
• Render check on GitHub web UI

Related

• docs: clarify #37 close reason — supersession by #44 chain-solve? (sister from #48 diagnosis) #49 (parent — usecase-routing.md row 27 add)
• cluster: chain follow-ups (#49 docs cross-link + #53 jq regex anchor) #58 (PR that landed row 27)

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