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`.

tests/plugins/test_kanban_dashboard_plugin.py

@@ -1833,3 +1833,49 @@ def test_run_metadata_secondary_styling():
     assert "max-height" in meta_decl
     assert "overflow: auto" in meta_decl
     assert "color: var(--color-muted-foreground)" in meta_decl
+
+
+def test_run_metadata_uses_native_collapse():
+    """Metadata panel uses <details>/<summary> for zero-JS collapse.
+
+    Native <details> means the browser handles state — no event handlers,
+    no React-state coupling, accessible by default (keyboard navigable,
+    screen-reader announces the disclosure state). Default-open state is
+    decided per-render based on payload length.
+    """
+    js = _dashboard_dist_path("index.js").read_text(encoding="utf-8")
+    # Element must be <details> / <summary>, not plain <div>s.
+    assert 'h("details"' in js
+    assert 'h("summary"' in js
+    # The open prop is computed from json length (collapsed when verbose).
+    assert "open: !collapsed" in js or "open:!collapsed" in js
+    assert "json.length > 300" in js
+
+
+def test_run_metadata_skips_empty_object():
+    """Empty `{}` metadata renders nothing — no useless labeled block.
+
+    `r.metadata && {} && ...` would render a "Metadata" labeled block
+    containing just `{}`, which is visual noise. The render predicate now
+    also checks Object.keys(r.metadata).length > 0.
+    """
+    js = _dashboard_dist_path("index.js").read_text(encoding="utf-8")
+    assert "Object.keys(r.metadata).length > 0" in js
+
+
+def test_run_metadata_disclosure_indicator_styled():
+    """Native disclosure marker is hidden + replaced with a CSS-only chevron.
+
+    Browsers render an OS-specific arrow next to <summary> by default. For a
+    consistent look across OSes the hermes dashboard hides that marker and
+    renders a CSS ::before chevron that rotates on [open]. Pin it so a
+    future CSS rebuild can't silently lose it (which would put two markers
+    side-by-side on Firefox/WebKit).
+    """
+    css = _dashboard_dist_path("style.css").read_text(encoding="utf-8")
+    # Default markers suppressed.
+    assert "list-style: none" in css
+    assert "::-webkit-details-marker" in css
+    # CSS-only chevron present + animates on open state.
+    assert ".hermes-kanban-run-meta-block[open]" in css
+    assert "rotate(90deg)" in css