Mark a list column that continues off screen

[tdwd]

Both dashboard columns already scroll — window keeps the cursor visible and slides only when it has to. Nothing said so. A list that fits and one that is cut looked identical, and arrowing past the edge was the only way to find out which you were looking at.

A clipped edge is now the ellipsis truncate already appends, so it means the same thing it means everywhere else in the frame.

 PROJECTS                        …
                                   project-07
 ▸ project-00                      project-08
   project-01                      project-09
   project-02                    ▸ project-10
   project-03                      project-11
   project-04                      project-12
   project-05                      project-13
 …                                …

Decisions

No count. The project list's lines are projects; the sidebar's are thirds of a session card. One number cannot be right in both columns, so the marker says "more this way" and the edge it sits on says which way.

One glyph, not a new one. util.go:23 already uses … for "cut off, there is more". A second symbol for the same fact is a second thing to learn.

The heading still scrolls away. Making it sticky costs two rows permanently, and the marker now tells you that you are scrolled. That is a real trade with no obvious winner, so it is left alone rather than decided quietly.

Three things the markers must not do, each pinned

• Cover the cursor. They cost the first and last row, and a cursor behind one would make the arrows look like they had stopped working. window centres focus, so the row it is on is never an edge when a marker goes there — TestWindowNeverMarksOverTheCursor checks every focus at heights 3 to 9.
• Edit the list. The slice window returns aliases its input, so marking in place would replace real rows in the caller's own list. TestWindowDoesNotEditTheList.
• Appear when they should not — on a list that fits, or on a column too short to spare the rows. Below three lines a clipped column would be all marker and no content.

Verification

Rendered at both call sites rather than trusted from unit tests: the marker lands in the cursor's own column in the project list, and replaces a card line cleanly in the sidebar with the selected card intact between them.

Marking was disabled and the suite rerun. TestWindowMarksAClippedEdge fails; the three guard tests pass, which is correct for guards.

Second commit

Describe both outcomes of x in the README — PR #12 gave x a modal and updated the key table, but left the prose under Sessions and worktrees describing only closing. Docs only.