Align local settings with guided EPP setup

[Hou (SciencePotato)]

Summary

Configuration-only follow-up to #16, based on current main. Aligns the shared local settings sample and guidance with the environment variables already generated by guided setup.

• Include the canonical provider/channel/region/authentication settings and EPP_OUTBOUND_CLIENT_ID / EPP_OUTBOUND_MI_CLIENT_ID.
• Preserve the complete selected send URL and exact provider scope; distinguish calling-app IDs, provider API IDs and managed-identity client IDs.
• Document migration from the earlier optional-JWT setting names without changing runtime authentication.
• Explain which setup metadata is informational and why cloud host settings and secrets should not be copied wholesale into local settings.

Scope

Only docs/local.settings.sample.json and docs/ONBOARDING.md change. No runtime, tests, dependencies, provider profile, setup script, private local settings or Azure resources are changed. The old JWT branch and PR #15 remain untouched; this does not port its runtime changes or alter API-key/JWT policy.

Validation

• Parsed local.settings.sample.json and verified every Values entry is a string.
• Compared sample keys against the setup PowerShell AST; all 13 provider/outbound settings are present and obsolete JWT keys are absent.
• Populated the sample in memory with the Soprano setup profile and verified the existing JavaScript config reader preserves all canonical fields, full URL and scope.
• Cross-checked the matching Python and .NET config readers.
• Editor diagnostics and git diff --check are clean.
• No live delivery, Azure mutation or production verification performed.