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/tools/task-pickup.ts

@@ -322,6 +322,7 @@ export function createTaskPickupTool(api: OpenClawPluginApi) {
           groupId,
           issueId: issue.iid,
           issueTitle: issue.title,
+          issueUrl: issue.web_url,
           role,
           model: dispatchResult.modelAlias,
           sessionAction: dispatchResult.sessionAction,