feat: Add documentation files including Code of Conduct, Contributing guidelines, and Security policy; update Markdown linting configuration and scripts

SECURITY.md

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+# Security Policy
+
+## Reporting a vulnerability
+
+If you believe you’ve found a security vulnerability in EchoHub (for example: auth bypass, token leakage, file upload validation bypass, RCE, etc.), please **do not** open a public GitHub issue.
+
+Preferred: use GitHub’s private vulnerability reporting ("Report a vulnerability"):
+
+- https://github.com/HueByte/EchoHub/security/advisories/new
+
+If that link is unavailable for your account, contact the maintainer via GitHub:
+
+- https://github.com/HueByte
+
+## What to include
+
+- A clear description of the issue and potential impact
+- Reproduction steps or a proof-of-concept
+- Affected versions / commit SHA
+- Any relevant logs (with secrets removed)
+
+## Disclosure
+
+I’ll acknowledge receipt, investigate, and work on a fix. Please avoid publicly disclosing details until a fix is available.