refactor(tiles): Ctrl+Tab and Alt+[ / ] cycle tiles through one helper

nextSession and prevSession each carried the same grid branch (cycleTile
from the active session, a human selection, skip the tab walk). It is now
_cycleTileFocus(delta) in tile-grid.js, next to the other focus moves;
both call it optionally, so a page or harness without tile-grid.js walks
the tabs as before.

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

src/web/public/app.js

@@ -9063,12 +9063,8 @@ class CodemanApp {
   }
 
   nextSession() {
-    // With the tile grid open, Ctrl+Tab and Alt+] cycle through the tiles.
+    // With the tile grid open, Ctrl+Tab and Alt+[ / Alt+] cycle through the tiles.
-    if (this._tileGrid?.open) {
+    if (this._cycleTileFocus?.(1)) return;
-      const id = window.CodemanTileGrid.cycleTile(this._tileGrid.ids, this.activeSessionId, 1);
-      if (id) this.selectSession(id);
-      return;
-    }
     if (this.sessionOrder.length <= 1) return;
 
     const currentIndex = this.sessionOrder.indexOf(this.activeSessionId);
@@ -9077,11 +9073,7 @@ class CodemanApp {
   }
 
   prevSession() {
-    if (this._tileGrid?.open) {
+    if (this._cycleTileFocus?.(-1)) return;
-      const id = window.CodemanTileGrid.cycleTile(this._tileGrid.ids, this.activeSessionId, -1);
-      if (id) this.selectSession(id);
-      return;
-    }
     if (this.sessionOrder.length <= 1) return;
 
     const currentIndex = this.sessionOrder.indexOf(this.activeSessionId);

src/web/public/tile-grid.js

@@ -608,6 +608,19 @@ Object.assign(CodemanApp.prototype, {
     if (id) this.selectSession(id);
   },
 
+  /**
+   * Ctrl+Tab / Alt+] (delta 1) and Alt+[ (delta -1) while the grid is open: a
+   * human selection of the next tile in reading order, wrapping. Returns
+   * whether the grid took the chord (it is open), so the tab walk is skipped.
+   */
+  _cycleTileFocus(delta) {
+    const grid = this._tileGrid;
+    if (!grid?.open) return false;
+    const id = window.CodemanTileGrid.cycleTile(grid.ids, this.activeSessionId, delta);
+    if (id) this.selectSession(id);
+    return true;
+  },
+
   /** Removes the focused tile (the session keeps running); a neighbour takes focus. */
   removeFocusedTile() {
     const grid = this._tileGrid;