docs(readme): add FAQ comparing OpenChrome with other browser MCPs

[shaun0927]

Summary

Closes #612 by adding a FAQ: Comparison with Other Browser MCPs section to the README, addressing the three questions raised by @fulldecent while evaluating OpenChrome against Chrome DevTools MCP and Firefox DevTools MCP:

• Multi-client tab safety — documents that OpenChrome uses stable CDP targetIds (not visible indexes) plus workerId / profileDirectory isolation, so tabs from one client cannot clobber another's.
• Interactive login — documents the two-mechanism approach: persistent-profile headless for "log in once, automate forever," with the lazy-launched headed fallback reserved for initial login / WAF-blocked sites.
• Focus stealing — explains why the recurring-popup pattern does not occur in OpenChrome (headless by default, tabs_create never calls bringToFront(), headed fallback is lazy and reused — at most one focus grab per server lifetime).

The new section sits between Anti-Bot & Turnstile Support and Benchmarks so it flows naturally from the 3-tier fallback explanation that Q2/Q3 build on.

Changes

• README.md: +42 lines, one new section. Docs-only, no code changes.

Test plan

• Verify Markdown renders correctly in the GitHub preview
• Cross-check internal terminology (workerId, profileDirectory, tabs_create, targetId) matches the code
• Confirm issue reference (#612) links to the correct thread
• Ensure section placement preserves existing navigation flow