feat: add check_subsession and an exploratory read_subsession transcript reader

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.

src/shared/apiTypes.ts

@@ -63,7 +63,8 @@ export interface PiWebConfigValues {
   spawnSessions?: boolean;
   /**
    * Beta: when true, LLMs can start tracked child sessions via the
-   * spawn_subsession / list_subsessions / read_subsession tools. Off by default
+   * spawn_subsession / list_subsessions / check_subsession / read_subsession
+   * tools. Off by default
    * while the capability stabilizes. Requires spawnSessions to be enabled.
    */
   subsessions?: boolean;