Bundle CLI and agent skill installation

[MildyNora]

What changed

• Bundles the complete Network Sandbox skill with every Linux and macOS release archive.
• Adds one-command Homebrew installation for the CLI and skill.
• Links the installed skill into Codex and general agent discovery directories.
• Replaces the Python preflight helper with direct dependency-free CLI checks.
• Adds a brief Anaconda-style quick start, lifecycle, rollback behavior, and real exit-code contract to the main README.
• Bumps the release to 0.6.2.

Why

Network Sandbox is an agent workflow, not just a binary. The previous download-only README did not bind the skill that instructs agents to protect active connectivity, and it did not explain the basic environment lifecycle.

Validation

• Skill structure validation passed.
• Rust formatting and lint checks passed.
• All 30 Rust tests passed.
• macOS Apple-silicon release build passed.
• Bundled archive contains the executable, SKILL.md, references, and agent metadata.
• Binary preflight reports Network Sandbox 0.6.2 and a ready native macOS backend.