feat: refine context into an AI-ready daily snapshot

[pentaoa]

context previously used the host timezone, hid the next class during an active class, conflated empty results with unavailable sources, and exposed weather/AQI only at verbose level without observation timestamps. This change makes the default live context a usable daily snapshot for AI assistants.

• Pin date/time to Asia/Shanghai and separate snapshot generation time from the reference instant. Return public academic-day flags, parity and makeup metadata.
• Add structured current, next and today's course instances using the existing ICS holiday/makeup rules; support upcoming classes before teaching starts and mark incomplete timetable coverage.
• Include weather/AQI at normal level with source, observation time, freshness and explicit US EPA scale. Keep library opening status at verbose level.
• Run academic and environmental reads concurrently, bound public environmental requests, preserve empty/error/partial/not-requested states, and return available data when the calendar fails.
• Reject live requests for non-today dates; retain non-live calendar previews. Update README and changelog.

Validation: build and all 320 tests passed. The 21 context/calendar tests also pass with TZ=UTC, including Shanghai midnight, active-plus-next classes, makeup dates, pre-term schedules, missing periods and stale observations. No campus mutation is performed by this feature.