Added flight schedule search feature

• Introduced scripts/schedule.py for searching scheduled flights between airports.
• Now supports both live flight tracking and schedule lookup via AviationStack API (optional, with API key).
• SKILL.md updated: covers both live tracking (OpenSky) and flight schedule features (AviationStack), explains API usage, and clarifies supported airport code formats.
• Added helpful fallback: without an API key, schedule search provides direct links to external flight search engines.
• Project description updated to reflect new scheduling capabilities.