docs+test: document HERMES_WEBUI_VISIBLE_SESSION_LIMIT; probe it in a subprocess

- README env table + .env.example gain the new setting (default 20, fallback
  semantics), pinned by test_setting_is_documented.
- tests/test_visible_session_limit_env.py no longer importlib.reload()s
  api.models in the shared pytest process (which recreated its module-level
  locks/caches/classes behind other modules' backs); each case now imports
  api.models in a fresh interpreter with a scrubbed env, matching
  test_issue3283_profiles_config_import_order.

.env.example

@@ -132,6 +132,11 @@
 # Extra sources for the notes drawer (see docs for the expected format).
 # HERMES_WEBUI_EXTERNAL_NOTES_SOURCES=
 
+# Sidebar recency window: how many recent sessions are listed (default: 20).
+# Also caps how many delegated subagent children can nest at once; raise it
+# for wide fan-outs. Invalid or non-positive values fall back to 20.
+# HERMES_WEBUI_VISIBLE_SESSION_LIMIT=20
+
 # Directory scanned for WebUI plugins (default: ~/.hermes/plugins).
 # HERMES_WEBUI_PLUGINS_DIR=~/.hermes/plugins