[docs] Add explicit URL link to #44 in usecase-routing.md row 27 (verify follow-up from #49)

[kiki830621]

Problem

From verification of #49 (PR #58):
「Row 61 plain text #44 chain-solve 沒給 issue URL。GitHub auto-linker 在 web UI 會 render,raw markdown viewer 不會。建議補 [#44](url) 達 cross-link 一致性」
— Source: team:regression + codex

usecase-routing.md row 27 + blockquote 中:

• #37 用 [#37](https://...issues/37) ✓
• #46 用 [#46 multi-root chain](https://...issues/46) ✓
• #44 用 plain text #44 chain-solve ✗

GitHub web UI 會 auto-link #NN,但 raw markdown(GitHub mobile / IDE preview / raw file fetcher)不會。為了 archaeology 一致性,補 explicit URL。

Type

documentation (cosmetic cross-link)

Priority

P3 (cosmetic — auto-linker covers most viewers)

Strategy

plugins/issue-driven-dev/references/usecase-routing.md row 61 (line containing #44 chain-solve):

- ... 
- `#44 chain-solve` → ...
+ - ...
+ - [`#44 chain-solve`](https://github.com/PsychQuant/issue-driven-development/issues/44) → ...

(具體 line 與 surrounding context 由實作時 inspect 確認)

Acceptance

• Row 27 / blockquote 內所有 #NN reference 一致 explicit URL
• Render correctness on raw markdown view

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)