story-kit: merge 166_story_add_done_column_to_pipeline_board

Add Done column to pipeline board. Adds the 'done' stage to
PipelineState, exposes it via the WebSocket and REST API, and
renders a Done column in the frontend pipeline board view.

Squash merge from feature/story-166_story_add_done_column_to_pipeline_board.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/LozengeFlyContext.test.tsx

@@ -13,6 +13,7 @@ function makePipeline(overrides: Partial<PipelineState> = {}): PipelineState {
 		current: [],
 		qa: [],
 		merge: [],
+		done: [],
 		...overrides,
 	};
 }