Organize documentation by moving requirements (HighLevelReqs, Component-*,
lmxproxy_protocol) to docs/requirements/ and test infrastructure docs to
docs/test_infra/. Updates all cross-references in README, CLAUDE.md,
infra/README, component docs, and 23 plan files.
Expand SMTP configuration with OAuth2 Client Credentials support for Microsoft 365,
connection timeout, and max concurrent connections. Single email per send with all
recipients in BCC. Plain text only. Classify SMTP errors: transient (4xx/connection)
to S&F, permanent (5xx) returned to script. No app-level rate limiting.
