Add FAQ / Troubleshooting section to README

[vidhartbhatia]

Add FAQ and Troubleshooting sections to README covering:

• "Why isn't it tracking?" (run /clear)
• "Where's my data?" (OS data dir paths)
• "Is my data synced anywhere?" (no, local-only unless git push enabled)
• "Can I use this without Copilot CLI?" (no)
• Common install/auth issues