Split subsession inspection into two tools: check_subsession gives a quick
glance (status + latest assistant output), while read_subsession reads through
a child's transcript with role/content filters, full-content substring search
(including tool-call args), optional per-value maxChars truncation that flags
clipped parts, includeToolArgs, and pagination.
Filtering and search run on full untruncated content; truncation is an explicit,
caller-owned final projection (no default) so a narrow read never silently hides
a match. Empty page-windows are distinguished from zero-match results.
