Ingest a multi-tab Excel workbook (Sarah's MASTER_DUEDATES_2026_v37.xlsx). Parse every sheet via openpyxl, classify likely-clients / likely-deadlines / unknown, surface ambiguous tabs + colored cells for the Sarah-teaches-agent flow. Writes a staging xlsx-import.session.json — never overwrites clients.json or deadlines/*.csv until the user confirms with [Apply].
