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/plugin.py

@@ -0,0 +1,63 @@
+"""Employees plugin main class.
+
+Blueprint-only plugin: a read-only employee directory lookup over a separate HR
+database. No model (the directory is an external DB) and no AssetType. The
+connection helper (employee_connection) stays core infrastructure in shopdb.api,
+shared with search and the notifications shopfloor feed.
+"""
+
+import json
+import logging
+from pathlib import Path
+from typing import List, Dict, Optional, Type
+
+from flask import Flask, Blueprint
+
+from shopdb.plugins.base import BasePlugin, PluginMeta
+
+from .api import employees_bp
+
+logger = logging.getLogger(__name__)
+
+
+class EmployeesPlugin(BasePlugin):
+    """Employees plugin - read-only HR directory lookup."""
+
+    def __init__(self):
+        self._manifest = self._load_manifest()
+
+    def _load_manifest(self) -> Dict:
+        """Load plugin manifest from JSON file."""
+        manifest_path = Path(__file__).parent / 'manifest.json'
+        if manifest_path.exists():
+            with open(manifest_path, 'r') as f:
+                return json.load(f)
+        return {}
+
+    @property
+    def meta(self) -> PluginMeta:
+        """Return plugin metadata."""
+        return PluginMeta(
+            name=self._manifest.get('name', 'employees'),
+            version=self._manifest.get('version', '1.0.0'),
+            description=self._manifest.get(
+                'description',
+                'Read-only employee directory lookup'
+            ),
+            author=self._manifest.get('author', 'ShopDB Team'),
+            dependencies=self._manifest.get('dependencies', []),
+            core_version=self._manifest.get('core_version', '>=0.1.0,<1.0.0'),
+            api_prefix=self._manifest.get('api_prefix', '/api/employees'),
+        )
+
+    def get_blueprint(self) -> Optional[Blueprint]:
+        """Return Flask Blueprint with API routes."""
+        return employees_bp
+
+    def get_models(self) -> List[Type]:
+        """No models - the directory is an external database."""
+        return []
+
+    def init_app(self, app: Flask, db_instance) -> None:
+        """Initialize plugin with Flask app."""
+        logger.info(f"Employees plugin initialized (v{self.meta.version})")