fix(workbench): align investigation diagram controls

src/styles.css

@@ -1353,6 +1353,12 @@ button.agent-history-session-trigger:not(:disabled):active {
 .investigation-diagram .node { cursor: pointer; }
 .investigation-diagram .node:hover { filter: brightness(0.97); }
 
+.investigation-diagram [data-streamdown="mermaid-block"] {
+  --diagram-control-rail-inset: 1.0625rem;
+  --diagram-control-group-gap: 0.5rem;
+  --diagram-zoom-rail-height: 7.625rem;
+}
+
 /* Match Button's compact icon-sm size, including controls portalled to body. */
 button[type="button"]:has(> .investigation-diagram-control-icon) {
   --diagram-control-size: 2rem;
@@ -1398,14 +1404,37 @@ div:has(> button > [data-investigation-diagram-control="zoom-in"]),
 
 .investigation-diagram [data-streamdown="mermaid-block"] > div:first-child { display: none; }
 .investigation-diagram [data-streamdown="mermaid-block"] > div:has(> [data-streamdown="mermaid-block-actions"]) {
+  position: absolute;
+  z-index: 20;
+  inset-block-start: auto;
+  inset-block-end: calc(
+    var(--diagram-control-rail-inset) + var(--diagram-zoom-rail-height) +
+      var(--diagram-control-group-gap)
+  );
+  inset-inline-start: auto;
+  inset-inline-end: var(--diagram-control-rail-inset);
   height: auto;
   margin-top: 0;
 }
 
+.investigation-diagram div:has(> button > [data-investigation-diagram-control="zoom-in"]) {
+  inset-inline-start: auto;
+  inset-inline-end: 0.5rem;
+}
+
 button[type="button"]:has(> [data-investigation-diagram-control="close"]) {
   position: absolute;
-  border-color: var(--surface-border);
+  inset-block-start: 1.3125rem;
+  inset-inline-end: 1.3125rem;
+  border-color: transparent;
   background: var(--surface-control);
+  box-shadow:
+    0 0 0 4px var(--surface-control),
+    0 0 0 5px var(--surface-border);
+}
+
+button[type="button"]:has(> [data-investigation-diagram-control="close"])::after {
+  inset: -5px;
 }
 
 @media (hover: hover) {
@@ -1424,6 +1453,13 @@ button[type="button"]:has(> .investigation-diagram-control-icon):active:not(:dis
   transform: scale(0.98);
 }
 
+button[type="button"]:has(> [data-investigation-diagram-control="close"]):active:not(:disabled) {
+  box-shadow:
+    0 0 0 4px var(--surface-control),
+    0 0 0 5px var(--action-border-pressed),
+    var(--action-shadow-secondary-pressed);
+}
+
 button[type="button"]:has(> .investigation-diagram-control-icon):focus-visible {
   outline: 2px solid var(--action-focus-ring);
   outline-offset: 2px;