feat(tiles): open and fill tiles by work, then input, then recency

The Tiles button with no stored grid opened the first tabs, so the oldest
sessions got tiles while the busy ones were left out. It now ranks the
open sessions: working first (the most recently started turn first, keyed
off lastSubmitAt only, since a working pane's activity stamp is always
"now"), then the ones needing input (the red and yellow tab alerts), then
everything else by most recent activity, tab order breaking ties.

The ranking is pure (rankTileSessions in constants.js) and reuses the home
screens' classification (_mobileOverviewState) and stamps. It feeds the
toggle's fresh set, a count picked in the menu, Ctrl/Cmd+click and an open
split's fill. The active session is still always included and focused,
detached sessions are never tiled, and a page without the classifier falls
back to tab order. The vm harness now loads mobile-overview.js so the app
tests exercise the real classifier.

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

test/mocks/tile-grid-vm.ts

@@ -1,7 +1,8 @@
 /**
  * @fileoverview A `vm` harness for the tile grid (tile-grid.js) with the real
  * app around it: constants.js + app.js + terminal-ui.js + terminal-split.js +
- * tile-grid.js in one context, a small fake DOM (just what the grid touches) and a fake
+ * tile-grid.js + mobile-overview.js (the session state classifier the grid's
+ * ranking reuses) in one context, a small fake DOM (just what the grid touches) and a fake
  * TerminalTile that records what the grid asks of it.
  *
  * `makeGridApp()` returns an app instance with everything around the grid that
@@ -295,7 +296,7 @@ const context = vm.createContext({
 });
 vm.runInContext(
   `${read('constants.js')}\n${read('app.js')}\n${read('terminal-ui.js')}\n${read('terminal-split.js')}\n` +
-    `${read('tile-grid.js')}\n` +
+    `${read('tile-grid.js')}\n${read('mobile-overview.js')}\n` +
     'globalThis.__CodemanApp = CodemanApp;',
   context
 );