feat: include ticket URL in task pickup announcements (#22)

- Add issue URL to announcement in dispatch.ts
- Add issueUrl field to workerStart notification event
- Update notification message to include URL
- Both automated notifications and manual announcements now show full ticket URL

Example output:
🔧 Spawning DEV (medior) for #7: Create TODO.md...
🔗 https://github.com/laurentenhoor/devclaw/issues/7

lib/dispatch.ts

@@ -266,7 +266,7 @@ export async function dispatchTask(
       ? "🔍"
       : "🔧";
   const actionVerb = sessionAction === "spawn" ? "Spawning" : "Sending";
-  const announcement = `${emoji} ${actionVerb} ${role.toUpperCase()} (${modelAlias}) for #${issueId}: ${issueTitle}`;
+  const announcement = `${emoji} ${actionVerb} ${role.toUpperCase()} (${modelAlias}) for #${issueId}: ${issueTitle}\n🔗 ${issueUrl}`;
 
   return {
     sessionAction,