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/base.html

@@ -55,15 +55,10 @@
             </a>
           </li>
           <li class="nav-item">
-            <a class="nav-link {% if request.path == '/emails' %}active{% endif %}" href="/emails">
+            <a class="nav-link {% if request.path in ['/emails', '/email-log'] %}active{% endif %}" href="/emails">
               <span>✉</span> Emails
             </a>
           </li>
-          <li class="nav-item">
-            <a class="nav-link {% if request.path == '/email-log' %}active{% endif %}" href="/email-log">
-              <span>📋</span> Log
-            </a>
-          </li>
           <li class="nav-item">
             <a class="nav-link {% if request.path == '/team' %}active{% endif %}" href="/team">
               <span>👥</span> Team