Employee directory: note it varies per site + add self-hosted option

- Flag the employee directory as the integration most likely to differ per
  site; USB (cmmc_usb) is standardized and rarely needs adaptation.
- Add "Option B: stand up a self-hosted directory" with the canonical employees
  table DDL, for sites with no HR database. In-app management (CRUD/CSV import)
  noted as a possible future enhancement.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/usb/README.md

@@ -9,6 +9,12 @@ The plugin's own reference tables (`usbdevicetypes`, `usbdevices`,
 `usbcheckouts`) live in the main app database; only the live check-in/out data
 is in `cmmc_usb`.
 
+> **This schema is typically standardized across sites** - the `cmmc_usb`
+> check-in/out solution is the same deployment everywhere, so the tables below
+> usually match as-is and no adaptation is needed. The view recipe at the end is
+> a fallback for the rare site that differs. (Contrast the employee directory,
+> which genuinely varies per site.)
+
 ## Connection
 
 Credentials resolve **settings-first, then environment**, except the password