The terminal's limbs screen watches or stops watching a milestone with `w`, as MCP `watch` does #7236

For: the owner follows a milestone from the terminal. It puts one item in their feed and sends one push when it lands.

watchMilestone (src/actions/watches.ts:21) is reached from the page (/:owner/:name/milestones/:number/watch) and MCP watch. src/tui/screens/limbs.ts binds raise, lower, take back and undo, but not watch.

Done:

• [ ] On a milestone row, w toggles watching, and the row shows a mark while watched. A person only: an agent gets MILESTONE_WATCH_PERSON_ONLY's words.
• [ ] A new test/tui-milestone-watch.test.ts covers: watch, the mark shows and the feed has the item, unwatch then both gone, and an agent refused.

Files: src/tui/screens/limbs.ts, one test. Risk: low.