Phase 2 of the pluggable cron-scheduler refactor. Still no call-site changes;
this wires up provider SELECTION with a hard safety net.
Task 2.1: cron.provider config key (hermes_cli/config.py), empty = built-in.
Additive key — deep-merge picks it up into existing configs with no version
bump (verified: load_config() yields the key on a pre-existing config.yaml).
Task 2.2: plugins/cron/__init__.py — discovery machinery cloned near-verbatim
from plugins/memory/__init__.py, retargeted at CronScheduler /
register_cron_scheduler. Bundled (plugins/cron/<name>/) + user
(/plugins/<name>/) dirs, bundled wins collisions. The built-in is
NOT discovered here — it's core, so the fallback can't be removed.
Task 2.3: resolve_cron_scheduler() in cron/scheduler_provider.py — reads
cron.provider and ALWAYS degrades to built-in (missing / unavailable / load
error / typo all fall back with a warning). cron can never be left without a
trigger.
Deviation from plan: the plan's resolver snippet used cfg_get("cron.provider")
(dotted-string form). The real cfg_get signature is cfg_get(cfg, *keys,
default=) — corrected to cfg_get(load_config(), "cron", "provider", default=""),
matching plugins/memory/__init__.py:349. Tests monkeypatch load_config (not
cfg_get) so the real traversal runs.
Tests: default key empty, discovery returns list, unknown load returns None,
and the four resolver paths (empty→builtin, no-section→builtin,
unknown→builtin, unavailable→builtin, available→used). Full tests/cron/: 453
passed; config suite green (additive key, no migration break).
