[codex] Add security policy

SECURITY.md

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+# Security Policy
+
+TaskBounty takes security issues seriously. If you discover a vulnerability in the smart contracts, frontend, or supporting tooling, please report it privately so maintainers can investigate and coordinate a fix before public disclosure.
+
+## Supported Versions
+
+The project is still evolving quickly, so security fixes are currently focused on the default branch and the latest release line.
+
+| Version | Supported |
+| --- | --- |
+| `main` | Yes |
+| Latest tagged release | Yes |
+| Older tags and unmaintained forks | No |
+
+## Reporting a Vulnerability
+
+Please do not open a public GitHub issue for an unpatched security bug.
+
+Instead, send a private report to the maintainers with:
+
+- a short summary of the issue and affected area
+- clear reproduction steps or a proof of concept
+- expected impact, including whether funds, identities, or contract state could be affected
+- any suggested mitigation or patch direction, if you have one
+- your preferred contact details for follow-up
+
+Use GitHub Security Advisories if enabled for the repository. If advisories are not available, open a private channel with the maintainers before sharing exploit details.
+
+## Response Expectations
+
+TaskBounty maintainers aim to:
+
+- acknowledge new reports within 3 business days
+- provide a status update within 7 business days
+- work with the reporter on validation, severity, and remediation timing
+- publish a coordinated disclosure after a fix is available or compensating controls are in place
+
+Response times may be longer for low-severity reports or during major release work, but maintainers should still acknowledge receipt and share next steps.
+
+## Coordinated Disclosure
+
+Please give maintainers reasonable time to investigate and release a fix before publicly disclosing a vulnerability. If the issue affects on-chain assets or live deployments, coordinated disclosure is especially important because remediation may require both code and operational changes.
+
+## Security Scope
+
+Security reports are especially valuable when they involve:
+
+- incorrect payout, escrow, or refund flows in the Soroban contracts
+- authorization or role bypasses
+- unsafe assumptions around deadlines, disputes, or submission approval
+- wallet integration or transaction-signing issues
+- secrets exposure, dependency compromise, or deployment misconfiguration
+
+Reports that only cover style issues, non-actionable best-practice notes, or unsupported third-party forks may be closed without a formal security response.