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>

docs/wiki/The-Dashboard.md

@@ -15,7 +15,7 @@ page says so and names the setting.
 | **Header, left**   | The "C" logo (goes home) and the session list, unless you moved it to the sidebar.       |
 | **Header, right**  | Status chips and panel buttons, most of them off by default.                            |
 | **Center**         | The terminal for the active session, or the home screen when nothing is selected.        |
-| **Bottom toolbar** | Run, Stop, Run Shell, the case picker, and the instance counters.                        |
+| **Bottom toolbar** | Run, Stop, Run Shell, the case picker, and the instance counter.                         |
 | **Overlays**       | Panels and modals: Respawn, Cron, Subagents, File Viewer, Settings.                      |
 
 ## Session list layout