[Feature Request] Add `--global` (`-g`) flag to `install` and support global path lookup

[jmjoy]

Description

I would like to propose adding a --global (or -g) flag to the agent-browser install command and updating the browser detection logic for other commands like open.

New Proposed Logic

1. Installation: Adding --global will install Chrome into a system-wide directory accessible by all users.
2. Detection (The "Fall-through" Lookup): Commands that require a browser (e.g., agent-browser open) should follow a prioritized search order:
   • Step 1: Check the user's local directory (current behavior).
   • Step 2: If not found locally, check the global/system-wide directory.
   • Step 3: If still not found, prompt the user to run the install command.

Motivation

This ensures a seamless experience in shared environments. A system administrator can perform a single global installation, and all users on the machine will be able to use agent-browser open immediately without needing to install their own local copies or manually configure paths.

Proposed CLI Usage

# To install globally (may require elevated privileges)
agent-browser install --global

# To use (will automatically find the global installation if local is missing)
agent-browser open