A pull's opened, reviewed and merged posts fold into one timeline post that shows its latest state #5808

An X-style feed shows one post per thing. Today one pull can fill three or four rows of the timeline.

Done when:

• [ ] Within the window the feed reads, pull.opened, pull.reviewed and pull.merged for the same pull become one post. It sits where the newest event ranks and reads by its latest state, for example merged a pull request: <title> · opened 3 h ago, 2 reviews.
• [ ] A failed run on that pull keeps its own post, because it is urgent.
• [ ] The chip counts and the cursor stay correct. A test pages through a feed of 30 folded pulls and finds each one exactly once.
• [ ] MCP feed folds the same way.

Files: src/feed/feed.ts, src/feed/items.ts, test/.