Add missing SPDX headers, security policy, and reduce dependency vulnerabilities

[Copilot]

Repository audit revealed missing license headers, lack of security documentation, and multiple dependency vulnerabilities.

Changes

License Compliance

• Added SPDX-License-Identifier headers to 17 JavaScript files across both test suites
• Files: config.js, erc721.js, hardhat.config.js, all utils, and HIP test files

Security Documentation

• SECURITY.md: Vulnerability reporting process, test key warnings
• KNOWN_ISSUES.md: Current vulnerability status, mitigation guidance
• Explicitly documented that private keys in repo are test-only and publicly known

Contribution Guidelines

• CONTRIBUTING.md: Development workflow, coding standards, PR process

Dependency Fixes

• evm-gas-schedule-compatibility-regression: Reduced vulnerabilities from 13 to 9
  • Eliminated all critical severity issues
  • Remaining vulnerabilities documented with mitigation notes
• system-contract-testing: Remaining vulnerabilities in dev dependencies (hardhat, @hashgraph/sdk) require breaking changes; documented in KNOWN_ISSUES.md

Verification

• Code review: 0 comments
• CodeQL scan: 0 alerts
• No functional changes introduced

Original prompt

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