feat(tiles): the grid holds at most 6 tiles (owner decision 7)

Six was tested smooth on the owner's desktop; nine missed the headless
frame bar and is untested on real hardware. TILE_GRID_MAX (constants.js)
is now 6 and stays the one cap every limit reads; the layout table gets
its own bound, TILE_LAYOUT_MAX = 9, so the 7 to 9 layouts keep working
(unreachable) and going back to nine is that one line.

Every way in stops at the cap: opening, addTile, a tile's +, a session
Run makes, Ctrl/Cmd+click, the picker, "Open group as tiles", and a stored
grid with more ids (it comes back as its first six, focus kept only if it
survives, a dropped zoom cleared, row fractions that no longer match the
3x2 reset). The limits now go through one helper, _tileGridLimit(), whose
texts say which limit binds: "Up to 6 tiles" / "The grid holds at most 6
tiles" when it is the cap, "This window fits N" when it is the window.

Docs: decision 7 and an as-built entry in the spec, CLAUDE.md, the
invariants, the wiki's Tile Grid and Dashboard pages.

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

docs/wiki/Tile-Grid.md

@@ -1,6 +1,6 @@
 # Tile Grid
 
-Watch and drive up to nine sessions at once, side by side in one window. Each tile is a
+Watch and drive up to six sessions at once, side by side in one window. Each tile is a
 full live terminal: it reads, it takes your keystrokes, and it shows at a glance whether
 its agent is working, idle, or waiting on you.
 
@@ -32,7 +32,8 @@ button in the header, beside Split, and enables `Ctrl+Shift+G`.
   joins it. Sessions started elsewhere (an agent, another device, a cron job) do not.
 
 The layout follows the tile count: 1x1, 2x1, three side by side on a wide screen (else a
-2x2 with one empty slot), 2x2, 3x2, 3x3.
+2x2 with one empty slot), 2x2, 3x2. The grid holds at most six tiles, fewer when the
+window is too small for six; the picker and a tile's + say which limit applies.
 
 ## A tile