fix(triage): recognize level-2 headings in issue bodies

[FenjuFu]

Summary

• What changed? parseIssueBody in .github/scripts/issue-triage-lib.ts now treats both ## and ### headings as section boundaries (/^#{2,3}\s+(.+)$/gm).
• Why is this needed? Issue forms render their labels as ###, but hand-written issues often use ##. Those issues were reported as missing every required section and routed to triage/needs-info with confidence 1/5, even when the content was complete (e.g. [Feature] Namespace 支持 24 小时单次邀请链接 #911, including after /retriage).

Validation

• Backend tests passed (N/A, no backend change)
• Frontend typecheck/build passed (N/A, no frontend change)
• OpenAPI SDK regenerated or checked when API contracts changed (N/A)
• Smoke test run when relevant

Ran analyzeIssue against the current body of #911, before and after the change, plus a form-style ### body to confirm nothing changes for form issues:

deno check .github/scripts/issue-triage.ts
deno run -A <script calling parseIssueBody/analyzeIssue on the #911 body>
# before: route=needs-info, missing=[problem, proposed solution], confidence=1
# after:  route=deferred,   missing=[],                           confidence=5
# ### form body: sections parsed exactly as before

Risk

• User-facing impact: triage results only. Issues written with ## headings get the same parsing as form issues.
• Deployment or migration impact: none. Existing triage comments are refreshed on the next rescore or /retriage.
• Rollback approach: revert this commit.

A ### sub-heading inside a ## section now starts its own section. That matches how form-based bodies were already split, and the required-field checks only look up the top-level names (problem, proposed solution, summary, ...).

Notes

• Related issue: Closes [Bug] Issue triage ignores level-2 headings and marks filled issues as needs-info #914
• Follow-up work: none
• Docs or operator runbooks updated when behavior changed: N/A

[FenjuFu]

Superseded by #916, which carries the same fix plus regression tests.