Pin the chat scroll fix: constrain the middle column flex chain

Why: the chat page could not scroll once messages exceeded the viewport
and the input bar was pushed off-screen (missing min-height:0 on the
column flex chain .main->.chat->.chat-body).
Evidence: combo make test green with the advanced frontend gitlink;
headless-Chromium before/after measurements and four layout variants
recorded in the task card.
Impact: middle column only; also ignore /data-test/ local runtime data
so precheck does not report BUSY on the user's test artifacts.
Refs: Agent-System-Frontend a54022c (feature/chat-scroll-fix).

.gitignore

@@ -1,4 +1,5 @@
 /bin/
 node_modules/
 /data/
+/data-test/
 .zcode/plans/