Add one-line bundled installer

[MildyNora]

What changed

• Adds a POSIX installer that detects the supported platform, downloads the latest archive, verifies SHA-256, installs the CLI, and links the bundled skill into Codex and general-agent skill directories.
• Adds the installer to future GitHub releases.
• Keeps Homebrew first in the README and provides the installer as a package-manager-free fallback.

Why

Homebrew itself can reject formula installation on beta macOS when Xcode and Command Line Tools lag the operating-system preview. Users still need a reliable one-command path that installs both the binary and agent workflow without compiling anything.

Validation

• Shell syntax check passed.
• Isolated installation downloaded and verified the public v0.6.2 release.
• Installed CLI reports netsandbox 0.6.2.
• Both agent skill links resolve to a bundle containing SKILL.md and agent metadata.
• Existing user-managed skill directories are preserved.