Merge #428: one toolbar instance stepper, not two

Only the stepper half of #428 lands here. Its other half, the phone case
picker search, shipped separately in #488 (1.33.1) and is kept as it is on
master, so every file of this merge starts from ours and only the stepper
changes are ported onto it:

- index.html: the second `− 1 +` group (#shellCount) after Run Shell is gone.
- session-ui.js: incrementShellCount/decrementShellCount are removed and
  runShell() reads the toolbar's one stepper through _readTabCount(), the
  helper master grew since #428 was opened (same clamp and absent-element
  fallback as #428's _toolbarInstanceCount()).
- run-mode-ui tests stub #tabCount instead of #shellCount, plus #428's
  "toolbar instance count" block adapted to _readTabCount() and a markup check.
- wiki: "the instance counter", singular.

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

src/web/public/index.html

@@ -700,11 +700,9 @@
           <button class="btn-toolbar btn-enter" onclick="app.sendEnterKey()" title="Send Enter">
             Enter
           </button>
-          <div class="tab-count-group" title="Instance count">
-            <button class="tab-count-btn" onclick="app.decrementShellCount()">−</button>
-            <input type="number" id="shellCount" class="tab-count-input" value="1" min="1" max="20" readonly>
-            <button class="tab-count-btn" onclick="app.incrementShellCount()">+</button>
-          </div>
+          <!-- Run Shell had a second, identical instance-count stepper here. The
+               toolbar carried two of them side by side, so it is gone and Run
+               Shell reads the one above (#tabCount) like the Run button does. -->
           <div class="case-select-group">
             <div class="case-combobox" id="quickStartCasePicker">
               <input