feat: support non-root installation in install.sh

[TimPietruskyRunPod]

Summary

The current install.sh requires root/sudo and hardcodes the install path to /usr/local/bin/. This is a barrier for container environments, restricted user setups, and CI pipelines where root access isn't available.

Desired behavior

• Detect whether the user is root or non-root
• Root: install to /usr/local/bin/ (current behavior)
• Non-root: install to a user-space directory (e.g. ~/.local/bin, ~/bin) that's already in $PATH, or fall back to ~/.local/bin with guidance on adding it to $PATH
• Remove the jq dependency — use portable grep/sed for JSON parsing
• Add Linux arm64 support (currently only amd64)
• macOS: prefer Homebrew if available, fall back to binary download

Related PRs

• feat: portable, root-free installer refactor (dependency-free) #240 and feat: unified integration matrix with gated CI signaling #241 proposed this feature but need rework to be compatible with the current install.sh on main (after fix: correct download URL pattern in install script #235 was merged)

Notes

• The install script was recently updated in fix: correct download URL pattern in install script #235 (download URL fix) — any changes must be based on current main
• Keep it simple and dependency-free where possible