docs: Add comprehensive SECURITY.md policy

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 0.1.x   | :white_check_mark: |
+
+## Reporting a Vulnerability
+
+ShieldLend is a ZK-based private DeFi lending protocol that handles sensitive cryptographic operations and user funds. Security is our highest priority.
+
+### How to Report
+
+**Please DO NOT open public issues for security vulnerabilities.**
+
+Instead, report security issues privately via:
+
+- **Email**: security@shieldlend.xyz (preferred)
+- **GitHub Security Advisories**: [Create a private advisory](https://github.com/cryptosingheth/shieldlend/security/advisories/new)
+
+### What to Include
+
+When reporting a vulnerability, please provide:
+
+1. **Description**: Clear explanation of the vulnerability
+2. **Impact**: What could an attacker accomplish?
+3. **Steps to Reproduce**: Detailed instructions to demonstrate the issue
+4. **Affected Components**: Specific contracts, circuits, or files
+5. **Proof of Concept**: Code, test cases, or screenshots if applicable
+6. **Suggested Fix**: If you have recommendations
+
+### Response Timeline
+
+- **Acknowledgment**: Within 48 hours
+- **Initial Assessment**: Within 7 days
+- **Resolution Timeline**: Depends on severity
+  - Critical: 7-14 days
+  - High: 14-30 days
+  - Medium/Low: 30-60 days
+
+### Bug Bounty
+
+We are establishing a bug bounty program. Details will be announced soon. Critical vulnerabilities in the following areas are of particular interest:
+
+- **Circuit Logic**: Soundness or completeness bugs in Circom circuits
+- **Smart Contracts**: Reentrancy, access control, or fund drainage issues
+- **Cryptographic Implementation**: Pedersen hash, Merkle tree, or proof verification flaws
+- **Front-running/MEV**: Attacks on deposit/withdraw privacy
+- **ZK Verification**: Bypassing zkVerify attestation
+
+### Scope
+
+**In Scope:**
+- Solidity contracts in `/contracts`
+- Circom circuits in `/circuits`
+- Frontend proof generation in `/frontend/src/circuits`
+- zkVerify integration contracts
+
+**Out of Scope:**
+- Third-party dependencies (OpenZeppelin, Circomlib)
+- Issues in test code or documentation
+- Social engineering attacks
+- Already disclosed vulnerabilities
+
+### Safe Harbor
+
+We follow responsible disclosure principles:
+- Researchers will not be prosecuted for good-faith security research
+- We will not take legal action against researchers who follow this policy
+- We will publicly acknowledge your contribution (with your permission)
+
+## Security Considerations
+
+### For Users
+
+1. **Note Security**: Your deposit note (secret + nullifier) is your only access key. Never share it and store it securely offline.
+2. **Amount Privacy**: While ShieldLend hides the link between deposits and withdrawals, deposit amounts are visible on-chain during the initial transaction.
+3. **Timing Correlation**: Consider waiting between deposit and withdrawal to reduce timing-based deanonymization risks.
+4. **Gas Costs**: ZK proof generation and verification incur higher gas costs than standard DeFi transactions.
+
+### For Developers
+
+1. **Circuit Constraints**: Always verify constraint counts when modifying circuits
+2. **Trusted Setup**: The current implementation uses a development trusted setup. Production deployments require a secure multi-party computation ceremony.
+3. **Merkle Tree Depth**: Current depth supports 2^20 leaves. Monitor usage and plan for tree rotation if approaching capacity.
+4. **Nullifier Registry**: Ensure nullifier uniqueness is strictly enforced to prevent double-spending.
+
+## Audit Status
+
+| Component | Status | Auditor | Date |
+|-----------|--------|---------|------|
+| Circuits | :yellow_circle: Pending | TBD | - |
+| Contracts | :yellow_circle: Pending | TBD | - |
+| Frontend | :yellow_circle: Pending | TBD | - |
+
+*Last Updated: March 2026*
+
+## Contact
+
+- Security Team: security@shieldlend.xyz
+- Project Lead: [@cryptosingheth](https://github.com/cryptosingheth)
+- Emergency Contact: See repository maintainers
+
+---
+
+Thank you for helping keep ShieldLend and its users safe!