Legacy rows and inserts left sessions.active_at NULL while SessionSchema
required a number, so CLI GET /cli/sessions/:id failed Zod and resume
surfaced HTTP 500. Persist active_at on insert, harden hub read coerce,
and nullish-transform activeAt in SessionSchema (output stays number).
Fixes#1025
Co-authored-by: Debian <heavygee@oos-linux.in.lockhouse>
Co-authored-by: Cursor <cursoragent@cursor.com>