feat(ai-vterm): show [running] in picker and F9 redisplays last-used

The C-F9 project picker now flags projects whose claude buffer is alive with a " [running]" suffix on the abbreviated path. I added `cj/--ai-vterm-format-candidate` to compute the display name and routed the picker through it. Before the change, the picker showed every candidate identically, so you couldn't tell at a glance whether picking a project would attach to an existing session or start a fresh one.

F9 with two or more alive claude buffers used to open the project picker. That meant after toggling claude-A off, opening claude-B via C-F9, then toggling B off, the next F9 dropped into a picker rather than redisplaying B (the one you just toggled off). I renamed `redisplay-single` to `redisplay-recent` in `cj/--ai-vterm-dispatch` and broadened the trigger from "exactly one alive" to "one or more alive". F9 now redisplays the MRU claude buffer, so it consistently means "toggle THE claude I was last using". The project picker stays explicit on C-F9 for "start a different project", and M-F9 still picks among existing claudes.

2 new tests for the indicator (`format-candidate` flagged + unflagged), 2 dispatch tests renamed to match the new contract. 80 ai-vterm tests pass. Full make test green.

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