chore: migrate to ESLint 9 with flat config

[Copilot]

Summary

Migrates the project from ESLint 8 to ESLint 9.

Changes

• Updated eslint from ^8.52.0 to ^9.39.4
• Replaced @typescript-eslint/eslint-plugin and @typescript-eslint/parser (v6) with the unified typescript-eslint package (v8), which supports ESLint 9's flat config format
• Replaced .eslintrc.json + .eslintignore with eslint.config.js (flat config) — preserving all existing rules and ignores
• Added lint script to package.json

Verification

• npm run lint — passes with no errors
• npm run build — passes successfully
• CodeQL scan — no alerts