With `chronos` in plugins.enabled every start warned
"Failed to load plugin 'chronos': 'PluginContext' object has no attribute
'register_cron_scheduler'": collect_directory_manifests skipped the bundled
memory/context_engine/model-providers categories but not cron_providers, and a
user-installed cron provider under ~/.hermes/plugins/<name> was never
auto-coerced to kind=exclusive. Cron providers activate through
plugins.cron_providers (cron.provider config), so the general manager must only
record them, exactly like memory providers.
Fixes#62951
credit: @wdmason #100362 (slim redo; #100396 is the consolidated duplicate)
