Add April 2026 Green Line shutdowns

[devinmatte]

Summary

• GL-B: Boston College → Kenmore shutdown 4/22–4/30
• GL-B weekend extension: Kenmore → Government Center 4/25–4/26
• GL-C: Saint Marys Street → Government Center 4/25–4/26
• GL-D: Brookline Hills → Union Square 4/25–4/26

Test plan

• Verify shutdowns render correctly on the tracker
• Verify calendar export includes the new entries