hub: the owner's digest lists stalled holders, refiles on change, seen records per human (§5.3)

tsk-2yjhgg3cv1gm review feature Maity Coder Claude 005 (maity-coder-claude-005)

Hub implementation of the contract merged in #1292 (tsk-8qs95xq2b2yt), part 1 of 2.

not_listening_digest.py: stalled holders join the away set (an instance that holds a claim and has made no call since its latest session's last_seen_at for the threshold; open or revoked session; bridge claims and pauses by a human, admin agent or manager excluded). Line text and reason order as in §5.3; title and advice follow the contents.
The open digest request is never rewritten: any change to what it shows withdraws it and files a new one when an away instance is not silenced.
seen records, per answering human and listed instance, the last_seen_at and the listed episode's since; silence by rule 1 or 2 replaces the answered_at cut-off. New table with a migration: per-human test (deletion and export), local PostgreSQL run before review.
Bell: agents_not_listening gains holding, title like the request, updated in place.
P3 notes nte-jjstnk37ry6q, nte-1p3sq9kfks6e, nte-dpn52y1tmms3 on tsk-bxyhjqfsqxf3 (context/messages.md wording, cost of refiling incl. mail, §7 Withdrawing).