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

docs/PLUGIN_AUTHORING.md

@@ -769,7 +769,7 @@ Bundled workspace plugin pattern:
 
 Runtime host context contract:
 - Registered views receive a `context` object from the dashboard host (`PluginDashboardViewContext`).
-- Context includes the active `projectId`, current visible `tasks`, optional `workflowSteps`, and `openTaskDetail` for launching the native task detail flow.
+- Context includes the active `projectId`, current visible `tasks`, optional `workflowSteps`, `openTaskDetail` for launching the native task detail flow, and `openFile(path, options?)` for opening project-relative files in the dashboard's built-in file viewer.
 - Keep view-specific UI behavior in the plugin; treat host context as service/data injection only.
 
 Placement guidance: