feat: AI Agents management page with per-agent performance tracking

New Agents tab in the sidebar (Ops group) for viewing, editing, and
creating AI agent configurations:

Database (migration 026):
- ai_agents table: editable configs for each LLM agent (model, prompts,
  temperature, tokens, retries). source='system' for built-in,
  source='user' for custom. Seeds 3 system agents (Document Extractor,
  Event Classifier, Thesis Rewriter) using WHERE NOT EXISTS to never
  overwrite user edits across reinstalls.
- agent_performance_log table: per-invocation metrics (duration,
  confidence, retries, tokens, errors) linked to agent config.

API endpoints:
- GET/POST /api/agents — list and create agents
- GET/PUT/DELETE /api/agents/{id} — view, edit, delete (system agents
  can be edited but not deleted)
- GET /api/agents/{id}/performance — aggregated metrics (success rate,
  avg/p95 latency, confidence, token usage)
- GET /api/agents/{id}/performance/history — hourly time series

Frontend:
- AgentsPage with sidebar list + detail panel
- Agent detail: config display, system prompt viewer, performance
  dashboard with metrics cards and time-series chart
- Edit form: all config fields editable including system prompt,
  model, temperature, tokens, retries
- Create form: new user-defined agents with auto-slug generation
- System agents show blue badge, user agents show green badge

frontend/src/components/AppLayout.tsx

@@ -18,6 +18,7 @@ import {
   List,
   Globe,
   BarChart3,
+  Bot,
 } from 'lucide-react';
 
 interface NavItem {
@@ -42,6 +43,7 @@ const navItems: NavItem[] = [
   { to: '/ops/pipeline', label: 'Pipeline', icon: <Activity size={18} />, group: 'Ops' },
   { to: '/ops/ingestion', label: 'Ingestion', icon: <Download size={18} />, group: 'Ops' },
   { to: '/ops/model', label: 'Model Perf', icon: <Cpu size={18} />, group: 'Ops' },
+  { to: '/agents', label: 'Agents', icon: <Bot size={18} />, group: 'Ops' },
   { to: '/ops/coverage', label: 'Coverage', icon: <Radar size={18} />, group: 'Ops' },
   { to: '/analytics/query', label: 'SQL Explorer', icon: <Terminal size={18} />, group: 'Analytics' },
   { to: '/analytics/dashboards', label: 'Dashboards', icon: <LayoutDashboard size={18} />, group: 'Analytics' },