[Feature] Install Script

[michaelmcnees]

Problem

Installation is manual and depends on your platform

Proposed solution

Write an install script that will auto-detect the user's OS, dependencies, prompt for a database URL, init a config file, add the binary to the user's path, etc.

[michaelmcnees]

Shipped in this milestone: packages/site/public/install.sh — curl-pipe-bash install script with OS/arch detection, checksum verification, /usr/local/bin or ~/.local/bin fallback, and PATH hint. MANTLE_INSTALL_YES=1 and MANTLE_VERSION env vars supported. Config wizard scoped out as a non-goal (no DB URL prompt — that belongs in mantle init). Closes #46.