Delete the Anthropic OAuth module, remove it from the built-in provider
registry, strip the OAuth client branch from the Anthropic streaming
provider, and replace the daemon orchestrator's token refresh with a
simple ANTHROPIC_API_KEY requirement.
Anthropic access is now API key or local Claude Code CLI only.
Closes#3952
Split flat docs/ into user-docs/ (guides, config, troubleshooting) and
dev/ (ADRs, architecture, extension guides, proposals). Updated
docs/README.md index to reflect new paths.
