fix(triage): support level-2 issue headings

[dongmucat]

Summary

• Allow issue triage to parse both level-2 (##) and level-3 (###) section headings.
• Prevent complete issues written with level-2 headings from being routed to triage/needs-info.
• Add regression coverage for level-2, level-3 Issue Form, and mixed heading bodies.

Validation

• Targeted Deno tests passed (3 passed, 0 failed)
• Deno format check passed
• Deno lint passed
• Deno type check passed
• git diff --check passed
• API contracts unchanged; no OpenAPI update required
• No frontend, backend Java, database, or deployment changes

Commands run:

npx --yes deno@2 test --config .github/scripts/deno.json .github/scripts/issue-triage-lib_test.ts
npx --yes deno@2 fmt --config .github/scripts/deno.json --check .github/scripts/issue-triage-lib.ts .github/scripts/issue-triage-lib_test.ts
npx --yes deno@2 lint --config .github/scripts/deno.json .github/scripts/issue-triage-lib.ts .github/scripts/issue-triage-lib_test.ts
npx --yes deno@2 check --config .github/scripts/deno.json .github/scripts/issue-triage-lib.ts .github/scripts/issue-triage-lib_test.ts

Risk

• User-facing impact: correctly classifies complete issues using ## headings.
• Deployment or migration impact: none.
• Rollback approach: revert commit 906a5e6c.

Notes

• Closes [Bug] Issue triage ignores level-2 headings and marks filled issues as needs-info #914
• Closes ISSUE-179
• Follow-up work: none.
• Docs or operator runbooks: no update required; behavior now matches accepted issue authoring formats.

[FenjuFu]

感谢补上回归测试。改动与 #915 一致，额外覆盖了 Issue Form 的 ### 和 ##/### 混用两种情况，比 #915 更完整。LGTM，合入后我会关闭 #915。