feat(kanban-dashboard): native <details> collapse + skip empty metadata

Two follow-up improvements to Tranquil-Flow's metadata-panel restyle.
Both stay within the parent PR's "tone down the panel" scope.

1. Native <details>/<summary> collapse for verbose metadata.

   The parent PR consciously deferred this ("adding native expand/collapse
   would be the next step but requires UX agreement"). The default they
   asked for is straightforward: collapsed when the rendered JSON exceeds
   300 chars (the threshold where the max-height: 8.5rem cap actually
   starts mattering), expanded otherwise. <details>/<summary> is the right
   primitive — zero JS, browser-handled state, accessible by default
   (keyboard-navigable, screen-reader announces the disclosure state),
   and survives any react-state churn for free.

   The OS-default disclosure marker is suppressed (list-style: none +
   ::-webkit-details-marker hidden) and replaced with a CSS ::before
   chevron that rotates 90deg on the [open] attribute, so the look is
   consistent across Firefox/WebKit/Blink without the double-marker
   that would otherwise appear on the platforms that still render the
   default triangle.

2. Skip rendering when metadata is an empty object.

   `r.metadata && ...` truthy-checks, but `{}` is truthy in JS — so a
   completed task with no actual metadata would render a "Metadata"
   labeled disclosure block containing literal `{}`. Adds an
   Object.keys(r.metadata).length > 0 guard so empty payloads render
   nothing instead of an empty disclosure stub.

Tests: three new static-asset assertions covering the <details> shape,
the empty-object skip, and the suppress-default-marker + animated-chevron
CSS — all in `tests/plugins/test_kanban_dashboard_plugin.py`.

plugins/kanban/dashboard/dist/index.js

@@ -2397,12 +2397,18 @@
           r.error
             ? h("div", { className: "hermes-kanban-run-error" }, r.error)
             : null,
-          r.metadata
-            ? h("div", { className: "hermes-kanban-run-meta-block" },
-                h("div", { className: "hermes-kanban-run-meta-label" }, "Metadata"),
-                h("code", { className: "hermes-kanban-run-meta" },
-                  JSON.stringify(r.metadata, null, 2)),
-              )
+          (r.metadata && Object.keys(r.metadata).length > 0)
+            ? (function () {
+                var json = JSON.stringify(r.metadata, null, 2);
+                var collapsed = json.length > 300;
+                return h("details", {
+                    className: "hermes-kanban-run-meta-block",
+                    open: !collapsed,
+                  },
+                  h("summary", { className: "hermes-kanban-run-meta-label" }, "Metadata"),
+                  h("code", { className: "hermes-kanban-run-meta" }, json),
+                );
+              })()
             : null,
         );
       }),