fix(byoa): don't ack unread the wake digest never showed (#29)

snapshotUnread builds `seen` from EVERY inbox row, but caps the digest handed to
the engine with one GLOBAL tail (`lines.slice(-40)`). On a clean turn the daemon
then calls `ackSeen(token, seen)` — "ack everything this turn saw" — over the
full map.

So anything the tail evicted is marked read having never been shown. And because
/conversation/mark-read pins each conversation's cursor at its NEWEST message id
(the unread filter is `ROW(mm.created_at, mm.id) > ROW(lr_at, lr_id)`), those
messages can never reappear in a later wake digest. The contract comment right
above snapshotUnread says the opposite is intended: "Captured BEFORE the engine
runs so `ackSeen` advances exactly what THIS turn saw".

The wake prompt makes it worse by telling the model the truncated set is
complete — "Your unread messages (ALREADY FETCHED — no need to re-run `cumora
inbox`)" — so it has no cue to re-fetch and no count to notice a gap.

Measured against the old renderer:

  - a human DM that arrived BEFORE a 200-message group burst is evicted whole:
    the conversation does not appear in the digest at all, and is then acked.
  - 25 conversations x 2 unread (the offline-catch-up shape): 12 conversations
    vanish entirely from the digest while all 25 read cursors jump forward. The
    agent cannot even name what it lost.
  - 60 unread in one room: 20 dropped with no marker of any kind.

Spend the same 40-line budget PER CONVERSATION instead of globally, water-filling
quietest-first so a small conversation keeps all of its messages and the busy one
absorbs the slack. This mirrors what the cloud path already does — loadContext
gives every conversation with unread its own window, and turn.ts prints an
explicit unread count — so this brings BYOA in line rather than inventing a
policy.

Whatever still cannot fit is announced in place with its exact count and the
command that reads it, because this turn is about to mark it read: dropping
unread in silence is the part that makes the loss unrecoverable.

When everything fits, the digest is byte-for-byte what it was.