Extract employee directory into a plugin

Third core feature pulled into a plugin (blueprint-only, like slides). The
employee directory is a read-only lookup over a separate HR database.

- plugins/employees/: manifest (api_prefix /api/employees, no deps), api/ (moved
  blueprint, contract-pure: success/error/ErrorCodes + employee_connection all
  from shopdb.api), plugin.py (get_blueprint, get_models -> []).
- employee_connection STAYS core infrastructure in shopdb.api (config-driven
  external DB connector, shared by search + the notifications shopfloor feed). So
  no get_services needed and no contract change - the plugin owns the directory
  FEATURE, core owns the shared connector.
- Fixed a latent bug in the move: error paths used ErrorCodes.DATABASE_ERROR
  which does not exist -> ErrorCodes.INTERNAL_ERROR (so a directory outage now
  returns a clean 500 envelope instead of an AttributeError crash).
- De-cored: deleted shopdb/core/api/employees.py, removed from
  CORE_BLUEPRINT_NAMES + core/api/__init__ import/__all__. Registered in
  instance/plugins.json.

Pinned first: validation (400) + graceful-degrade (500) characterization tests;
the degrade test caught the DATABASE_ERROR bug and goes green with the fix.
184 tests pass, naming green, app boots 9 bundled plugins, endpoint verified live.

Plugin extractions complete: knowledgebase, slides, employees.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/employees/api/__init__.py

@@ -0,0 +1,5 @@
+"""Employees plugin API."""
+
+from .routes import employees_bp
+
+__all__ = ['employees_bp']