📸 Update README screenshots to showcase all four Studio modes

Replace three screenshots with four 24%-width images showing Commit Mode,
Code Review, Changelog, and Chat with Iris. Update alt text to describe
each mode specifically and revise caption to reflect the full range of
Studio capabilities.

Author: hyperb1iss

README.md

@@ -18,12 +18,13 @@
 </div>
 
 <div align="center">
-  <img src="https://raw.githubusercontent.com/hyperb1iss/git-iris/main/docs/images/git-iris-screenshot-1.png" alt="Git-Iris Screenshot 1" width="33%">
-  <img src="https://raw.githubusercontent.com/hyperb1iss/git-iris/main/docs/images/git-iris-screenshot-2.png" alt="Git-Iris Screenshot 2" width="33%">
-  <img src="https://raw.githubusercontent.com/hyperb1iss/git-iris/main/docs/images/git-iris-screenshot-3.png" alt="Git-Iris Screenshot 3" width="33%">
+  <img src="https://raw.githubusercontent.com/hyperb1iss/git-iris/main/docs/images/git-iris-screenshot-1.png" alt="Commit Mode" width="24%">
+  <img src="https://raw.githubusercontent.com/hyperb1iss/git-iris/main/docs/images/git-iris-screenshot-2.png" alt="Code Review" width="24%">
+  <img src="https://raw.githubusercontent.com/hyperb1iss/git-iris/main/docs/images/git-iris-screenshot-3.png" alt="Changelog" width="24%">
+  <img src="https://raw.githubusercontent.com/hyperb1iss/git-iris/main/docs/images/git-iris-screenshot-4.png" alt="Chat with Iris" width="24%">
 </div>
 
-_Iris exploring your codebase and generating contextual commit messages_
+_Iris crafting commit messages, reviewing code, generating changelogs, and chatting about your changes_
 
 ## 💜 Overview