Require explicit sources for all cluster functions
Remove implicit source defaults from cluster(), cluster_period(),
cluster_span(), and cluster_range(). All functions now require a
sources dict parameter, ensuring callers explicitly specify which
transcript sources to load.
This eliminates the confusing dual-layer default system where
_DEFAULT_INSTRUCTIONS provided one set of defaults while cluster
functions overrode them based on schedule type.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>