docs: add security policy (#826)

SECURITY.md

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+# Security Policy
+
+Thank you for helping keep Zerobyte and its users secure.
+
+Zerobyte is currently in `0.x`, and releases may include breaking changes between versions. For that reason, security fixes are only guaranteed for the most recent stable release line.
+
+## Supported Versions
+
+| Version                                       | Supported          |
+| --------------------------------------------- | ------------------ |
+| Latest stable `0.x` release                   | :white_check_mark: |
+| Older stable `0.x` releases                   | :x:                |
+| Pre-release builds (`-alpha`, `-beta`, `-rc`) | :x:                |
+| Development snapshots from `main`             | :x:                |
+
+Security fixes are generally released in the latest stable version only. If you are running an older release, you may be asked to upgrade before a fix is provided.
+
+## Reporting a Vulnerability
+
+Please do **not** report security vulnerabilities through public GitHub issues, discussions, or pull requests.
+
+Use one of these private channels instead:
+
+1. Preferred: GitHub private vulnerability reporting
+   https://github.com/nicotsx/zerobyte/security/advisories/new
+2. Alternative: contact the maintainer directly by email if an address is listed in the repository or GitHub profile
+
+When reporting, include as much of the following as you can:
+
+- affected Zerobyte version
+- deployment details, including whether you are using Docker, reverse proxies, or exposed ports
+- clear reproduction steps or a proof of concept
+- impact assessment and what an attacker could do
+- any relevant logs, screenshots, or configuration excerpts with secrets removed
+
+### What to expect
+
+- We aim to acknowledge new reports within 7 days.
+- We aim to provide status updates at least every 7 days while the report is being investigated.
+- If the report is accepted, we will work on a fix, coordinate disclosure, and publish a security advisory when appropriate.
+- If the report is declined, out of scope, or cannot be reproduced, we will explain why when possible.
+
+Please avoid public disclosure until a fix has been released and maintainers have had reasonable time to notify users.