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/client/src/components/settings/SettingsSessiondPanel.ts

@@ -67,7 +67,7 @@ export class SettingsSessiondPanel extends LitElement {
             >
             <span>Enable the <code>spawn_subsession</code> tools</span>
           </label>
-          <small>Beta: agents can start child sessions they stay attached to (<code>spawn_subsession</code>, <code>list_subsessions</code>, <code>read_subsession</code>) and are notified when a child finishes. Requires "Allow agents to start sessions". Off by default.</small>
+          <small>Beta: agents can start child sessions they stay attached to (<code>spawn_subsession</code>, <code>list_subsessions</code>, <code>check_subsession</code>, <code>read_subsession</code>) and are notified when a child finishes. Requires "Allow agents to start sessions". Off by default.</small>
         </div>
         <section class="effective-card" aria-label="Effective configuration summary">
           <h3>Effective after environment overrides</h3>