FN-6119: open CE docs in built-in file viewer

Expose the dashboard file viewer to plugin views and wire Compound Engineering artifacts to it.

- add an openFile callback to the dashboard plugin view context and pass through the app host implementation
- switch Compound Engineering artifact Open actions to the built-in file viewer with matching styling and coverage
- document the new plugin context capability and add a published changeset for the CLI package

Files changed:
 .changeset/ce-docs-built-in-viewer.md              |  5 +++
 docs/PLUGIN_AUTHORING.md                           |  2 +-
 packages/dashboard/app/App.tsx                     |  1 +
 packages/dashboard/app/plugins/types.ts            |  2 ++
 .../src/dashboard-interop.d.ts                     |  1 +
 .../src/dashboard/CompoundEngineeringView.css      | 16 +++++++++
 .../src/dashboard/CompoundEngineeringView.tsx      | 24 +++++++-------
 .../__tests__/CompoundEngineeringView.test.tsx     | 38 ++++++++++++++++++++++
 8 files changed, 76 insertions(+), 13 deletions(-)

Fusion-Task-Id: FN-6119

Fusion-Task-Lineage: 8feb461c-b1d2-4059-9aa1-ffc756d15196

plugins/fusion-plugin-compound-engineering/src/dashboard-interop.d.ts

@@ -24,6 +24,7 @@ declare module "@fusion/dashboard/app/plugins/types" {
     tasks: Task[];
     workflowSteps: WorkflowStep[];
     openTaskDetail: (task: Task | TaskDetail, initialTab?: DetailTaskTab) => void;
+    openFile: (path: string, options?: { workspace?: string; line?: number; col?: number }) => void;
     renderTaskCard?: (task: Task | TaskDetail) => ReactNode;
     addToast?: (message: string, type?: PluginToastType) => void;
     subscribePluginEvents?: (

plugins/fusion-plugin-compound-engineering/src/dashboard/CompoundEngineeringView.css

@@ -135,6 +135,22 @@
   opacity: 0.55;
 }
 
+.ce-artifact-open {
+  appearance: none;
+  background: none;
+  border: none;
+  color: var(--color-primary, #2563eb);
+  cursor: pointer;
+  font: inherit;
+  padding: 0;
+  text-decoration: underline;
+}
+
+.ce-artifact-open:hover,
+.ce-artifact-open:focus-visible {
+  color: var(--color-primary-hover, #1d4ed8);
+}
+
 .ce-artifact-error .ce-artifact-error-msg {
   color: var(--color-danger, #d23);
   font-size: 0.78rem;

plugins/fusion-plugin-compound-engineering/src/dashboard/CompoundEngineeringView.tsx

@@ -7,7 +7,6 @@ import { useArtifacts } from "./hooks/useArtifacts.js";
 import { useViewportMode } from "./hooks/useViewportMode.js";
 import { useCeSession, type CeSessionSubscribe } from "./hooks/useCeSession.js";
 import { useCeSessions, type CeSessionsSubscribe } from "./hooks/useCeSessions.js";
-import { getArtifactPreviewUrl } from "./hooks/api.js";
 import { CeFlow } from "./CeFlow.js";
 import { getStage, listStages, type CeStageDefinition } from "../session/stage-registry.js";
 import type { CeArtifactEntry, CeArtifactGroup } from "../artifacts/discovery.js";
@@ -170,14 +169,14 @@ function EmptyState({ onStart }: { onStart: () => void }) {
 
 function ArtifactRow({
   entry,
-  projectId,
   onSelect,
   selected,
+  openFile,
 }: {
   entry: CeArtifactEntry;
-  projectId?: string;
   onSelect: (id: string) => void;
   selected: boolean;
+  openFile?: PluginDashboardViewContext["openFile"];
 }) {
   if (entry.kind === "error") {
     return (
@@ -196,28 +195,28 @@ function ArtifactRow({
         <span className="ce-artifact-name">{entry.name}</span>
         <span className="ce-artifact-path">{entry.path}</span>
       </button>
-      <a
+      <button
+        type="button"
         className="ce-artifact-open"
-        href={getArtifactPreviewUrl(entry.id, projectId)}
-        target="_blank"
-        rel="noreferrer"
+        data-testid="ce-artifact-open"
+        onClick={() => openFile?.(entry.path)}
       >
         Open
-      </a>
+      </button>
     </li>
   );
 }
 
 function StageGroup({
   group,
-  projectId,
   onSelect,
   selectedId,
+  openFile,
 }: {
   group: CeArtifactGroup;
-  projectId?: string;
   onSelect: (id: string) => void;
   selectedId?: string;
+  openFile?: PluginDashboardViewContext["openFile"];
 }) {
   const empty = group.entries.length === 0;
   return (
@@ -236,9 +235,9 @@ function StageGroup({
             <ArtifactRow
               key={entry.id}
               entry={entry}
-              projectId={projectId}
               onSelect={onSelect}
               selected={selectedId === entry.id}
+              openFile={openFile}
             />
           ))}
         </ul>
@@ -261,6 +260,7 @@ export function CompoundEngineeringView(props: CompoundEngineeringViewProps) {
   // host doesn't supply it, the hook falls back to polling.
   const subscribePluginEvents = (props.context as PluginDashboardViewContext | undefined)
     ?.subscribePluginEvents;
+  const openFile = props.context?.openFile;
   const subscribe = useMemo<CeSessionSubscribe | undefined>(() => {
     if (!subscribePluginEvents) return undefined;
     return (sessionId, _projectId, onSessionEvent) =>
@@ -415,9 +415,9 @@ export function CompoundEngineeringView(props: CompoundEngineeringViewProps) {
             <StageGroup
               key={group.stage}
               group={group}
-              projectId={projectId}
               onSelect={setSelectedId}
               selectedId={selectedId}
+              openFile={openFile}
             />
           ))}
         </div>

plugins/fusion-plugin-compound-engineering/src/dashboard/__tests__/CompoundEngineeringView.test.tsx

@@ -119,6 +119,44 @@ describe("CompoundEngineeringView", () => {
     expect(screen.getAllByTestId("ce-group-empty").length).toBeGreaterThan(0);
   });
 
+  it("opens an artifact in the built-in file viewer via context.openFile", async () => {
+    const openFile = vi.fn();
+    listArtifacts.mockResolvedValue(
+      makeResult({
+        strategy: [
+          { kind: "artifact", id: "strategy:STRATEGY.md", stage: "strategy", path: "STRATEGY.md", name: "STRATEGY.md", size: 10, updatedAt: 1 },
+        ],
+      }),
+    );
+    render(
+      <CompoundEngineeringView
+        projectId="p1"
+        enabledOverride
+        context={{ openFile, tasks: [], workflowSteps: [], openTaskDetail: vi.fn() }}
+      />,
+    );
+
+    await screen.findByTestId("ce-artifact");
+    fireEvent.click(screen.getByTestId("ce-artifact-open"));
+    expect(openFile).toHaveBeenCalledWith("STRATEGY.md");
+  });
+
+  it("renders artifact open button without crashing when openFile is not in context", async () => {
+    listArtifacts.mockResolvedValue(
+      makeResult({
+        strategy: [
+          { kind: "artifact", id: "strategy:STRATEGY.md", stage: "strategy", path: "STRATEGY.md", name: "STRATEGY.md", size: 10, updatedAt: 1 },
+        ],
+      }),
+    );
+    render(<CompoundEngineeringView projectId="p1" enabledOverride />);
+
+    await screen.findByTestId("ce-artifact");
+    const open = screen.getByTestId("ce-artifact-open");
+    expect(open).toBeInTheDocument();
+    fireEvent.click(open);
+  });
+
   it("renders an error entry for an unreadable artifact (not a crash or silent drop)", async () => {
     listArtifacts.mockResolvedValue(
       makeResult({