Polish AgentFence repository landing page

[Adulph3]

Transforms the AgentFence repository landing page into a complete, security-focused project overview without changing production behavior.

Presentation

• adds an original dark-mode-friendly SVG banner at docs/assets/agentfence-banner.svg
• strengthens the hero, description, badges, and release/install links
• reorganizes the README for fast scanning and beginner-friendly setup

Documentation

• explains the real configuration-exposure problem and verified safety model
• documents supported Codex, Claude Code, Cursor, Kiro, VS Code, and generic MCP surfaces
• adds verified Linux, macOS, and Windows installation/checksum steps
• documents Quick Start, the actual CLI surface, exit-code precedence, CI usage, architecture, release integrity, status, limitations, security reporting, and contribution guidance
• preserves the Windows inherited-ACL warning and satisfies the repository documentation contract

Verified release facts

• current version: v0.1.0
• runtime: Node.js >=24 <25
• artifact: agentfence-0.1.0.tgz
• SHA-256: 4d5271aad1a7f56f66555752d6a3184643cac0213a712079af33d95709db278b
• distribution: GitHub Releases only; not npm

No scanner logic, rules, scoring, CLI behavior, package behavior, workflow, branch protection, release, or security control is changed.