Revise README for Claude Code installation and security

[GraceKellySS]

This pull request updates the README.md to improve installation instructions, enhance security guidance, and add troubleshooting and best practices for using Claude Code on a VPS. The changes focus on making the setup process more robust, recommending the native installer, strengthening SSH security, and introducing self-hardening features.

Installation and Setup Improvements:

• Recommends using the native installer for Claude Code instead of the deprecated npm method, and provides updated steps for installation, PATH configuration, and verification.
• Adds a note specifying tested distributions (Ubuntu 20.04+ / Debian 10+).

Security Enhancements:

• Provides a clearer, safer SSH hardening process using a dedicated drop-in config file, disables root login, and emphasizes using SSH keys only.
• Adds a new section on Claude Code self-hardening by creating a .claude/settings.json to deny access to common secret files, with guidance on team-wide and local configurations.
• Adds a dedicated Security Note section reminding users to review permissions and avoid running as root.

Troubleshooting and Best Practices:

• Updates troubleshooting for OAuth login, clarifies steps if login gets stuck, and provides guidance for resolving claude not found/PATH issues.
• Updates memory management advice to recommend using /compact and swap, not just restarting.

Documentation and Resource Formatting:

• Improves the Resources section by formatting it as a table for clarity.