Add troubleshooting section to README

[bntvllnt]

Problem

README covers happy path well but lacks error scenarios and FAQ.

Missing Content

• What happens if LOG_LEVEL is set to an invalid value?
• How to debug if PostHog adapter fails silently?
• How to verify logs are being captured in tests?
• Common setup mistakes

Acceptance Criteria

• Add Troubleshooting / FAQ section to README
• Cover top 3-5 questions consumers would have

Priority

Low — can add reactively as users file questions.