research: split pipeline diagnostics into research_pipeline.rs

Gates step failed on run#211 because research.rs grew to 1313 lines
(over the 1250 budget) after adding the pipeline_state handler in the
previous commit. Move the diagnostic into its own module — pure
extraction, no behavior change.

research.rs: 1313 → 1126 lines
research_pipeline.rs: new, 187 lines
lib.rs: route now points at routes::research_pipeline::pipeline_state

crates/cm-api/src/routes/mod.rs

@@ -19,6 +19,7 @@ pub mod orgs;
 pub mod planner;
 pub mod repos;
 pub mod research;
+pub mod research_pipeline;
 pub mod routines;
 pub mod sessions;
 pub mod skills;