The terminal's forks screen says where this repository stands against what it follows, and what the crew is doing about it, as MCP `following` does #7302

For: the owner of a fork who opens it in the terminal and wants to know if it is behind its original. Fork tree means a fork always knows its parent.

relationOf and convergeState are what MCP following answers. The terminal's forks screen (src/tui/screens/forks.ts) ranks the forks of a repository but says nothing of the repository's own parent.

Done:

• [ ] When the repository follows another, the forks screen's first line names it and says how far behind and ahead it is, in the same words as MCP following.
• [ ] A second line says what the crew is doing about it (an open update pull, a card, or nothing), only when there is something.
• [ ] enter on that line opens the original, if the viewer can read it.
• [ ] A repository that follows nothing shows no such line.
• [ ] A new test/tui-following.test.ts: a fork behind its parent says so; one in step says it is in step; an unfollowed repository shows nothing.

Files: src/tui/screens/forks.ts, one test. Risk: low; read only.