feat(agents): show the Changes pane only when it's useful, toggled from the task header (#421)

The Changes pane took a fixed column next to every agent conversation, including tasks that changed nothing. It is now shown while the task has changes (so it appears when the agent first edits a file) and toggled from a Changes button with the file count in the task header; once toggled, the choice is kept per task.

Co-authored-by: a-rookie-of-C-language <[email protected]>

src/features/agents/Session.module.css

@@ -3,7 +3,7 @@
   min-height: 0;
   min-width: 0;
   display: grid;
-  grid-template-columns: minmax(0, 1fr) 280px;
+  grid-template-columns: minmax(0, 1fr) auto;
   background: var(--bg-editor);
   color: var(--fg);
 }
@@ -558,6 +558,42 @@
   flex-direction: column;
   min-height: 0;
   min-width: 0;
+  width: 280px;
+}
+.changesCollapsed {
+  width: 28px;
+  align-items: center;
+  padding-top: 4px;
+}
+.changesCollapsed > * {
+  flex: none;
+}
+.changesRail {
+  flex: 1;
+  min-height: 0;
+  align-self: stretch;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 6px;
+  padding: 4px 0 8px;
+  border: none;
+  background: transparent;
+  color: var(--fg-subtle);
+  font: inherit;
+  cursor: pointer;
+}
+.changesRail:hover {
+  color: var(--fg-strong);
+}
+.changesRailLabel {
+  writing-mode: vertical-rl;
+  font-size: var(--fs-small);
+  letter-spacing: 0.02em;
+}
+.changesRailCount {
+  font-size: var(--fs-small);
+  color: var(--fg-muted);
 }
 .changesHeader {
   display: flex;