feat(sidebar): make the visible-session window size configurable

CLI_VISIBLE_SESSION_LIMIT is hardcoded to 20. That constant also bounds
how many delegated subagent children can render at once, since a child
only nests when its parent row is in the same payload, so operators
running wide delegate_task fan-outs cannot see all children without
editing the source.

Read it from HERMES_WEBUI_VISIBLE_SESSION_LIMIT via the existing
_env_int helper, keeping 20 as the default.

api/models.py

@@ -48,7 +48,11 @@ from api.agent_sessions import (
 from api.process_event_utils import stamp_message_source
 
 logger = logging.getLogger(__name__)
-CLI_VISIBLE_SESSION_LIMIT = 20
+# Size of the interactive sidebar recency window. Also bounds how many
+# delegated subagent children can be rendered at once, since a child only
+# nests when it wins a slot in this window. Override with
+# HERMES_WEBUI_VISIBLE_SESSION_LIMIT.
+CLI_VISIBLE_SESSION_LIMIT = _cfg._env_int("HERMES_WEBUI_VISIBLE_SESSION_LIMIT", 20)
 # How many messageful cron sessions to surface in the project-chip layer.
 # Needs to exceed CLI_VISIBLE_SESSION_LIMIT so older cron runs stay
 # addressable even when many newer non-cron sessions dominate the default