Adds an observation_scopes config key (and HINDSIGHT_RETAIN_OBSERVATION_SCOPES
env var) so retained memories can opt into per_tag / all_combinations /
custom scoping instead of Hindsight's default combined pass.
Threaded through _build_retain_kwargs so all three retain paths honor it:
auto-retain and flush-on-switch already use aretain_batch; the tool retain
path is switched from aretain to aretain_batch (functionally equivalent,
aretain just wraps a single-item batch) since aretain doesn't accept the
observation_scopes parameter.
a376ca0081