refactor(ci): extract shared helpers from description-check scripts

[rudra496]

Summary

The two description-check scripts — check-pr-description.js and check-issue-description.js — had three near-identical blocks of logic copied between them: markdown section extraction, fail-soft label management, and find-by-marker comment upsert/delete. Every fix had to be made twice, and the two copies had already drifted (the issue script escaped ? in Are you willing to implement this? as \\? while the PR script didn't).

This PR extracts the shared logic into .github/scripts/lib/:

• markdown.js — strip(text) and section(body, heading). section escapes regex specials in the heading internally so callers pass plain text (e.g. Are you willing to implement this? without pre-escaping the ?). This fixes the drift — both scripts now use the same escaping.
• labels.js — labelExists / addLabel / dropLabel / swapLabel, preserving the existing fail-soft discipline (never auto-create; warn and skip when absent).
• comment.js — findComment / upsertComment / deleteComment (find-by-marker, paginated lookup).

Both check scripts now require() these helpers instead of carrying their own copies. No behaviour change — the same API calls are made in the same order with the same fail-soft handling.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #2453

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)
• New feature (non-breaking — adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I ran node --check on all 5 files (2 scripts + 3 lib modules) — all pass.

How to Test

1. node --check .github/scripts/check-pr-description.js — passes.
2. node --check .github/scripts/check-issue-description.js — passes.
3. node --check .github/scripts/lib/markdown.js && node --check .github/scripts/lib/labels.js && node --check .github/scripts/lib/comment.js — all pass.
4. The CI workflows that call these scripts (ci.yml → "Check PR description" and issue-description-check.yml → "Check Issue description") will exercise the refactored code on this PR itself.

Visual / UI changes — REQUIRED if you touched anything that renders

Not applicable: CI/automation scripts only.

• I am not an LLM agent submitting a bulk PR. If you are, please open an issue describing the problem first — bulk auto-generated PRs that don't match the project's visual style are closed on sight, even when the underlying fix is correct.

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