Fix double notifications from duplicate event dispatch and listener registration

[Copilot]

Agent messages triggered duplicate notifications due to three concurrent bugs in the dashboard polling logic.

Root Causes

Duplicate event dispatch - Each message dispatched newAgentMessage event twice:

// Before
data.messages.forEach(message => {
  document.dispatchEvent(new CustomEvent('newAgentMessage', { detail: message }));
  document.dispatchEvent(new CustomEvent('newAgentMessage', { detail: message })); // duplicate
});

// After
data.messages.forEach(message => {
  document.dispatchEvent(new CustomEvent('newAgentMessage', { detail: message }));
});

Unchecked re-initialization - Multiple calls to initializeDashboard() registered duplicate event listeners. Added isInitialized guard.

Interval leak - startMessagePolling() created new intervals without clearing previous ones. Now calls clearInterval() before setInterval().

Changes

• public/js/dashboard.js - Removed duplicate dispatch, added init guard and interval cleanup
• __tests__/dashboard.test.js - Test coverage for all three bugs
• Documentation files for investigation and troubleshooting

Impact

• Notifications: 2× → 1× per message
• Eliminated memory leaks from uncleaned intervals and listeners
• ~50% reduction in event processing overhead

Original prompt

when an agent send a message we get double notifications could find out why is that happens?

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