Merges the working macOS setup from issue #5 into the Troubleshooting
section. Adds an 'Auth issues' subsection covering the common 401
'authentication failed' / 'No Anthropic credentials found' symptoms and
the Keychain-to-~/.claude/.credentials.json mirror step that fixes them.
The billing/routing section is separated out to keep auth failures
distinct from the 'third-party apps draw from extra usage' bug (#6),
which is a billing-classification issue, not an auth issue.
Closes#5.
