docs: add CONTRIBUTING, SECURITY, and Dependabot config

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in contributing!
+
+## Getting Started
+
+1. Fork the repository
+2. Create a feature branch: `git checkout -b feat/your-feature`
+3. Make your changes
+4. Commit using [Conventional Commits](https://www.conventionalcommits.org/): `feat:`, `fix:`, `chore:`, etc.
+5. Push and open a pull request
+
+## Reporting Issues
+
+Open a [GitHub Issue](../../issues) with a clear description and steps to reproduce.
+
+## Code Style
+
+Follow the existing conventions in the codebase.

SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+Please **do not** open a public GitHub issue for security vulnerabilities.
+
+Open a [GitHub Security Advisory](../../security/advisories/new) or email the maintainer directly.
+
+Include:
+- Description of the vulnerability
+- Steps to reproduce
+- Potential impact
+
+You will receive a response within 72 hours.