docs: align README hero tagline and subtitle with docs/index.md (#3581)

Match the README hero tagline to the docs landing hero and rewrite the
subtitle to reflect the four-pillar positioning (ready-to-use spec-driven
process or bring your own, extensible, community-driven, org-ready) rather
than framing everything around SDD.

Assisted-by: GitHub Copilot (model: Claude Opus 4.8, supervised)

Copilot-Session: da32794c-5044-406c-9338-12b3ffab49f4

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: mnriem

README.md

@@ -1,11 +1,11 @@
 <div align="center">
     <img src="./media/logo_large.webp" alt="Spec Kit Logo" width="200" height="200"/>
     <h1>🌱 Spec Kit</h1>
-    <h3><em>Build high-quality software faster.</em></h3>
+    <h3><em>Define what to build before building it — with any AI coding agent.</em></h3>
 </div>
 
 <p align="center">
-    <strong>An open source toolkit that allows you to focus on product scenarios and predictable outcomes instead of vibe coding every piece from scratch.</strong>
+    <strong>An open source toolkit for building high-quality software with any AI coding agent — a ready-to-use spec-driven process (or bring your own), endlessly extensible, community-driven, and built for your whole organization.</strong>
 </p>
 
 <p align="center">