docs: add first-screen result sentence

Per codex audit: this README opened with method-only framing, so
a recruiter scanning the first screen got pipeline scope but no
concrete result. Adds one sentence right after the methods line
surfacing the headline deliverables: 2,604 cells retained after QC,
5 immune populations, CD4/CD8 T-cell subclustering, CI-backed
reproducibility. Numbers all verifiable elsewhere in the README
(line 110: cells; lines 46, 110: clusters; line 118: T-cell
subclustering; CI badge).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Ekin-Kahraman

README.md

@@ -6,6 +6,8 @@
 
 Single-cell RNA-seq analysis pipeline in Python using [scanpy](https://scanpy.readthedocs.io/). Doublet detection, quality control, normalisation, dimensionality reduction, clustering with automated resolution selection, marker-based cell type annotation, PAGA trajectory inference, and T cell subclustering on human peripheral blood mononuclear cells.
 
+Retains 2,604 PBMCs, resolves 5 major immune populations plus CD4/CD8 T-cell subclusters, with CI-tested reproducibility.
+
 <p align="center">
   <img src="docs/umap_3d_rotation.gif" alt="3D UMAP rotation showing PBMC immune cell clusters" width="600">
 </p>