Fix YAML formatting for AAMAS 2026 dates

Add quotes around start and end dates for consistency with other conference entries.

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/data/conferences/aamas.yml

@@ -60,8 +60,8 @@
   city: Paphos
   country: Cyprus
   venue: Coral Beach Hotel & Resort, Paphos, Cyprus
-  start: 2026-05-25
-  end: 2026-05-29
+  start: '2026-05-25'
+  end: '2026-05-29'
   tags:
   - machine-learning
   - robotics