feat: Complete UX cleanup - Agent colors & streamlined navigation

Agent Color System:
- Added unique colors for each agent (orchestrator: purple, researcher: teal, negotiator: red, ar_manager: yellow)
- CSS variables and utility classes (.agent-*, .agent-badge-*, .agent-border-*)
- Tasks table now shows colored left border per agent
- Agent names displayed in their brand color

Navigation Simplification:
- Removed 'Email Log' from main menu (reduced clutter)
- Email Log now accessible via link in Emails page
- Active state for both /emails and /email-log on Emails nav item

Visual Improvements:
- Agent assignments now visually distinct at a glance
- Cleaner, more focused navigation menu
- Better information architecture

templates/tasks.html

@@ -30,7 +30,7 @@
             </thead>
             <tbody>
               {% for task in tasks %}
-              <tr>
+              <tr class="agent-border-{{ task.agent_key or 'default' }}">
                 <td style="color:var(--text-muted);">{{ task.id }}</td>
                 <td>
                   <strong>{{ task.title }}</strong>
@@ -49,7 +49,9 @@
                     </div>
                   {% endif %}
                 </td>
-                <td style="font-size:.8rem;">{{ task.assigned_agent }}</td>
+                <td style="font-size:.8rem;" class="agent-{{ task.agent_key or 'default' }}">
+                  <strong>{{ task.assigned_agent }}</strong>
+                </td>
                 <td>
                   {% if task.status == 'pending' %}
                     <span class="badge bg-warning">Pending</span>