feat(tiles): the grid is cells, and an empty cell can be any cell

Owner feedback 1: "so the empty tab doesnt always have to be the last
one! so I can move freely around and the empty tab can also be tab nr
4 or 3". This replaces the slot refusal of aecada8c.

grid.cells (a session id or null per cell) is now the one source of
truth; grid.ids is a getter deriving the tiles in reading order, so
everything that only wants the tiled sessions (focus neighbour,
cycling, the load queue's order, the picker, closeSession) is
unchanged. The shape still comes from the tile count and the cap
counts tiles, never empty cells.

- A tile dragged onto an empty cell moves there and leaves its own
  cell empty, nothing else moving (_moveTileToCell, through
  _reorderTiles: no remount, reconnect or reload; only a tile whose
  cell size changed fits). A tiled session's tab does the same; a tab
  of a session not tiled yet joins in the cell it is dropped on. Each
  slot knows its cell and reads "Drop a tab or a tile here".
- Move Tile goes to the adjacent cell: into it when empty, a swap when
  a tile is there (tileCellInDirection).
- Removing a tile leaves its cell empty; adding one takes the first
  empty cell. A shape change goes through fitTileCells: each tile keeps
  its row and column when all fit (2x2 growing to 3x2), else the tiles
  pack in reading order.
- Focus never lands on an empty cell: Alt+Shift+Arrows run over the
  cells, Ctrl+Tab and Alt+[ ] over the tiles.
- codeman:tile-grid stays ids only: its ids are the cells with null for
  an empty one. A reload brings the holes back when the shape is the
  same (a session gone since leaves its cell empty), another shape
  packs, the old packed format reads unchanged, and a followed
  #session= link keeps the holes.

Docs: the spec's as-built bullet (rewritten in place), the wiki's Tile
Grid page and Keyboard Shortcuts, the invariants and CLAUDE.md.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/wiki/Keyboard-Shortcuts.md

@@ -45,8 +45,8 @@ Anything you copy is cleaned on the way to the clipboard: each line loses the pa
 | --------------------------- | ------------------------------------------------------------ |
 | `Ctrl+Shift+G`              | Open or close the tile grid (needs the Tiles setting on).     |
 | `Alt+Shift+Arrows`          | Focus the tile to the left, right, above or below.            |
-| `Ctrl+Shift+Arrows`         | Move the focused tile: it trades places with that neighbour.  |
-| Drag a tile's header        | Move the tile: dropped on another tile, the two swap.         |
+| `Ctrl+Shift+Arrows`         | Move the focused tile one place: into an empty slot, or swap. |
+| Drag a tile's header        | Move the tile: onto another tile they swap, onto an empty slot it moves there. |
 | `Alt+Shift+Enter`           | Zoom the focused tile, or restore the grid.                   |
 | `Ctrl`+click / `Cmd`+click a tab | Add that session to the grid.                            |
 | Right-click the Tiles button | Choose which sessions to show as tiles.                  |

docs/wiki/Tile-Grid.md

@@ -65,18 +65,23 @@ whose agent exited inside its pane says so instead; close that session from `⋯
 ## Moving tiles
 
 Drag a tile by its header (anywhere but its buttons) onto another tile and the two trade
-places; the dropped tile takes the focus. Empty slots are for adding sessions (drop a tab
-there), so a tile cannot be dropped on one. Press `Escape` or let go anywhere else and nothing
-changes, not even which tile has the focus: a header focuses its tile when you click it, not
-when you press it.
+places. Drop it on an empty slot and it moves there, leaving its old place empty; nothing else
+moves, so the empty slot can be anywhere in the grid. The dropped tile takes the focus. Press
+`Escape` or let go anywhere else and nothing changes, not even which tile has the focus: a
+header focuses its tile when you click it, not when you press it.
 
-With the keyboard, `Ctrl+Shift+Arrows` moves the focused tile left, right, up or down: it
-trades places with the tile next to it (the one `Alt+Shift+Arrows` would focus) and keeps the
-focus.
+With the keyboard, `Ctrl+Shift+Arrows` moves the focused tile one place left, right, up or
+down: into the empty slot if that is the place, else trading places with the tile there. It
+keeps the focus.
 
 A moved tile takes the size of the place it lands in: column widths and row heights stay
-where you dragged the dividers. Tiles do not move while one is zoomed. The new order is
-saved with the grid.
+where you dragged the dividers. Tiles do not move while one is zoomed. Where everything is,
+the empty slot included, is saved with the grid and comes back on reload.
+
+Closing a tile leaves its place empty when the grid keeps its shape (six tiles to five), and a
+new tile takes the first empty place. When the number of tiles changes the grid's shape (four
+tiles to five is two columns to three), the tiles keep their places if they still fit, or line
+up again from the top left. `Alt+Shift+Arrows` and `Ctrl+Tab` never stop on an empty slot.
 
 ## Keys