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/api/client.test.ts

@@ -266,6 +266,7 @@ describe("ChatWebSocket", () => {
 			current: [],
 			qa: [],
 			merge: [],
+			done: [],
 		};
 		instances[1].simulateMessage({ type: "pipeline_state", ...freshState });