Add is-hangul-jamo-nieun-cieuc-present API utility

[ChaiXinLong-tech]

Closes #8629

/claim #8629

Summary

• Add isHangulJamoNieunCieucPresent() for detecting the Unicode Hangul choseong nieun-cieuc character (U+115C).
• Keep the helper dependency-free and scoped to a single API utility file.
• Update contributors/agents.json for this bounty claim.

Tests

• RED: strict TypeScript compile of the batch test files failed before helper modules existed.
• GREEN: work/agent-playground/node_modules/.bin/tsc.cmd --noEmit --strict --lib es2020 ... passed for the batch helper and test files.
• GREEN: compiled the batch helper tests to outputs/tmp-batch259/dist-green-run and executed 5 Node test files.