fix(#248): ARM64 Linux auto-fallback to Firefox + --browser CLI flag

[giulio-leone]

Closes #248

Problem

Chromium is not available for ARM64 Linux (aarch64). Playwright fails with 'Chromium distribution not found'.

Solution

1. Auto-fallback: When chromium is requested on ARM64 Linux, automatically use Firefox instead (with a warning)
2. --browser flag: New CLI flag to explicitly choose browser engine (chromium, firefox, webkit)
3. Env var: AGENT_BROWSER_BROWSER for configuration

Usage

# Explicit Firefox
agent-browser --browser firefox open https://example.com

# Auto-fallback on ARM64 Linux (no flag needed)
agent-browser open https://example.com  # Uses Firefox automatically

[vercel]

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