feat: add --local flag to install.sh for developer builds

Adds `--local` mode to install.sh that builds from local source instead
of downloading from GitHub. In local mode: requires Rust + Bun, builds
native modules from source via napi-rs, uses rsync to sync packages,
skips dependency auto-install and interactive prompts, disables
auto-update.

Usage: ./install.sh --local

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Tim Kersten