Migration: host.json + local.settings.json schema compatibility audit

Confirm `host.json` and `local.settings.json` schemas behave identically
in v5. Document any breaking changes and update the migration guide
accordingly.

## Definition of Done

- [ ] `host.json` schema compatibility audited
- [ ] `local.settings.json` schema compatibility audited
- [ ] Any breaking schema changes documented
- [ ] Migration guide updated

Compare the v5 behavior of the `host.json` and `local.settings.json` schemas first, then inspect the repository's migration guide. Record any breaking schema changes and update the migration guide; done means both schemas are audited and all identified changes are documented.

Written by the indexing model from the issue text.