docs: add SECURITY.md with private disclosure policy (fixes #290)

Co-Authored-By: Claude Opus 4.8 <[email protected]>

SECURITY.md

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+# Security Policy
+
+We take the security of PraisonAIUI seriously and appreciate responsible
+disclosure of vulnerabilities.
+
+## Supported Versions
+
+Security fixes are applied to the latest released version of PraisonAIUI.
+Please make sure you are on the most recent release before reporting.
+
+## Reporting a Vulnerability
+
+**Please do not open a public issue for security vulnerabilities.**
+
+Report privately through GitHub's **Private Vulnerability Reporting**:
+
+1. Go to the [Security tab](https://github.com/MervinPraison/PraisonAIUI/security)
+   of this repository.
+2. Click **"Report a vulnerability"** to open a private advisory.
+3. Include as much detail as you can:
+   - Affected component(s) and version.
+   - Steps to reproduce.
+   - Impact assessment.
+   - Any suggested fix or mitigation.
+
+If you are unable to use private reporting, you may open a public issue that
+contains **no technical specifics** and requests a private channel, and a
+maintainer will follow up.
+
+## What to Expect
+
+- We aim to acknowledge new reports within a few business days.
+- We will keep you informed as we investigate and work on a fix.
+- Once a fix is released, we are happy to credit you in the advisory (unless
+  you prefer to remain anonymous).
+
+Thank you for helping keep PraisonAIUI and its users safe.