Two changes to let non-DeepSeek providers use their own model IDs:
1. config.rs: add requested_model_for_provider() — DeepSeek providers
use strict normalize_model_name(); all others accept any non-empty
string via normalize_custom_model_id().
2. subagent/mod.rs: normalize_requested_subagent_model() now takes an
ApiProvider parameter and delegates to requested_model_for_provider.
Error messages name the active provider and list accepted model IDs
from model_completion_names_for_provider() instead of hardcoding
"Expected a DeepSeek model id".
parse_optional_subagent_model() keeps basic trimming-only validation;
provider-aware checks are deferred to the spawn path where the runtime
is available.
Hunter Bown
faeeeef59b