fix(mobile): show the Codex shift-arrow keys only on codex sessions

The two keys #408 adds to the mobile keyboard accessory bar send
Shift+Left and Shift+Right, which are Codex bindings (edit the last
queued message, step back through the prompt stack). They shipped on
both agent layouts, so a claude, pi, grok, omp, deepseek or gemini
session got two keys that do nothing. That was not only cosmetic: a tap
goes through sendNavKey(), which adds the session to
_echoPassthroughSessions and hands editing to plain PTY echo until Enter
or Ctrl+C, so on a phone a dead key also switched off the local echo
that makes typing feel instant there.

The reveal now follows the shape the 🧠 key already uses. The buttons
stay in both templates, carry an accessory-btn-codex marker class, and
are display:none in styles.css until the bar element carries
codex-enabled. The class has to live on the bar rather than on the keys
because setMode() rebuilds the buttons' innerHTML on every layout
switch. syncCodexKeys() toggles it from the active session's mode
(the same lookup _isShellSession() uses) and is called at init and from
refreshForActiveSession(), which selectSession() already invokes on
every switch. A session's mode is readonly on the server and fixed at
create, so no other event can change the answer; the welcome screen
(no active session) reads as not codex and hides the keys.

The frontend id-branching guard (test/cli-registry-no-id-branching.test.ts)
scans only src/**/*.ts, so the mode comparison in a public JS file is
in bounds, the same as the existing shell check beside it.

Tests: the new describe block in test/mobile-shell-keyboard.test.ts pins
the marker class in both templates, the CSS pair, the class for a codex
session in both layouts, its absence for claude/shell/pi/omp/deepseek,
the re-sync in both directions on a session switch, the no-session case,
and the init + refresh wiring. All six positive assertions fail without
the source change. README and the changeset now say the keys are
Codex-only.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/web/public/keyboard-accessory.js

@@ -4,8 +4,10 @@
  * Defines three exports:
  *
  * - KeyboardAccessoryBar (singleton object) — Quick action buttons shown above the virtual
- *   keyboard on mobile: arrow up/down, /init, Tab, Shift+Left/Right, paste, Esc, and dismiss (the extended
- *   bar adds /clear, /compact, Shift+Tab and more). Tab flushes any locally-buffered
+ *   keyboard on mobile: arrow up/down, /init, Tab, paste, Esc, and dismiss (the extended
+ *   bar adds /clear, /compact, Shift+Tab and more). Shift+Left/Right ship in both agent
+ *   layouts but are revealed only on Codex sessions (`codex-enabled` marker class on the
+ *   bar, synced on every session switch), since they are Codex bindings. Tab flushes any locally-buffered
  *   prompt text to the PTY before sending \t, so completion applies to what was typed.
  *   The paste button opens a dialog that handles both text paste and image attach
  *   (native picker + best-effort image paste, routed through app._uploadAndInsertImages).
@@ -661,8 +663,8 @@ const KeyboardAccessoryBar = {
       </button>
       <button class="accessory-btn" data-action="init" title="/init">/init</button>
       <button class="accessory-btn" data-action="tab" title="Tab">Tab</button>
-      <button class="accessory-btn" data-action="shift-left" title="Shift+Left (Codex: edit queued message)" aria-label="Shift+Left (Codex: edit queued message)">⇧←</button>
-      <button class="accessory-btn" data-action="shift-right" title="Shift+Right (Codex: prompt stack back)" aria-label="Shift+Right (Codex: prompt stack back)">⇧→</button>
+      <button class="accessory-btn accessory-btn-codex" data-action="shift-left" title="Shift+Left (Codex: edit queued message)" aria-label="Shift+Left (Codex: edit queued message)">⇧←</button>
+      <button class="accessory-btn accessory-btn-codex" data-action="shift-right" title="Shift+Right (Codex: prompt stack back)" aria-label="Shift+Right (Codex: prompt stack back)">⇧→</button>
       <button class="accessory-btn" data-action="paste" title="Paste from clipboard">
         <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">
           <path d="M16 4h2a2 2 0 0 1 2 2v14a2 2 0 0 1-2 2H6a2 2 0 0 1-2-2V6a2 2 0 0 1 2-2h2"/>
@@ -748,8 +750,8 @@ const KeyboardAccessoryBar = {
       <button class="accessory-btn" data-action="clear-input" title="Clear the current unsent input">&#x232B; All</button>
       <button class="accessory-btn accessory-btn-rmm" data-action="readmymind" title="Read My Mind: predict your next prompt">🧠</button>
       <button class="accessory-btn" data-action="tab" title="Tab">Tab</button>
-      <button class="accessory-btn" data-action="shift-left" title="Shift+Left (Codex: edit queued message)" aria-label="Shift+Left (Codex: edit queued message)">⇧←</button>
-      <button class="accessory-btn" data-action="shift-right" title="Shift+Right (Codex: prompt stack back)" aria-label="Shift+Right (Codex: prompt stack back)">⇧→</button>
+      <button class="accessory-btn accessory-btn-codex" data-action="shift-left" title="Shift+Left (Codex: edit queued message)" aria-label="Shift+Left (Codex: edit queued message)">⇧←</button>
+      <button class="accessory-btn accessory-btn-codex" data-action="shift-right" title="Shift+Right (Codex: prompt stack back)" aria-label="Shift+Right (Codex: prompt stack back)">⇧→</button>
       <button class="accessory-btn" data-action="shift-tab" title="Shift+Tab">⇧Tab</button>
       <button class="accessory-btn" data-action="effort-max" title="/effort max">Max</button>
       <button class="accessory-btn" data-action="ctrl-o" title="Ctrl+O">⌃O</button>
@@ -776,6 +778,9 @@ const KeyboardAccessoryBar = {
     // The 🧠 key is opt-in (`readMyMindEnabled`, synced): it ships in both
     // templates but stays display:none until the bar carries the marker class.
     this.syncReadMyMind();
+    // The ⇧←/⇧→ keys are Codex bindings: same shape, gated on the active
+    // session's mode instead of a setting.
+    this.syncCodexKeys();
 
     // Add click handlers — preventDefault stops event from reaching terminal
     this.element.addEventListener('click', (e) => {
@@ -819,6 +824,7 @@ const KeyboardAccessoryBar = {
   refreshForActiveSession() {
     this.clearCtrl();
     this._applyLayout(this._resolveMode());
+    this.syncCodexKeys();
   },
 
   /** Which layout the current state calls for. */
@@ -831,6 +837,11 @@ const KeyboardAccessoryBar = {
     return app.sessions?.get(app.activeSessionId)?.mode === 'shell';
   },
 
+  _isCodexSession() {
+    if (typeof app === 'undefined' || !app.activeSessionId) return false;
+    return app.sessions?.get(app.activeSessionId)?.mode === 'codex';
+  },
+
   /** Swap the button set in the DOM. */
   _applyLayout(mode) {
     if (!this.element || mode === this._mode) return;
@@ -1019,6 +1030,20 @@ const KeyboardAccessoryBar = {
     this.element.classList.toggle('rmm-enabled', enabled === true);
   },
 
+  /** Reveal the ⇧←/⇧→ keys only while the active session runs Codex. They are
+   *  Codex bindings (edit the last queued message / prompt stack back) and do
+   *  nothing in any other CLI, yet a tap still goes through sendNavKey(), which
+   *  hands the session to plain PTY echo for the rest of the prompt, so on a
+   *  phone a dead key would also switch off local echo. Same marker-class
+   *  shape as syncReadMyMind(): the class lives on the BAR because setMode()
+   *  rebuilds the buttons' innerHTML. Synced at init and on every session
+   *  switch (refreshForActiveSession); a session's mode is fixed at create, so
+   *  no other event can change the answer. */
+  syncCodexKeys() {
+    if (!this.element) return;
+    this.element.classList.toggle('codex-enabled', this._isCodexSession());
+  },
+
   /** Send a slash command to the active session.
    *  Sends text and Enter separately so Ink processes them as distinct events. */
   sendCommand(command) {

src/web/public/styles.css

@@ -11969,6 +11969,18 @@ kbd {
   display: inline-flex;
 }
 
+/* Keyboard-accessory ⇧←/⇧→ keys: Codex bindings (edit the last queued
+   message / prompt stack back), so they are revealed only while the active
+   session runs Codex. Same marker-class shape as the 🧠 key above, for the
+   same reason (the bar's innerHTML is rebuilt on every layout switch); the
+   class is synced from the active session's mode (keyboard-accessory.js). */
+.keyboard-accessory-bar .accessory-btn-codex {
+  display: none;
+}
+.keyboard-accessory-bar.codex-enabled .accessory-btn-codex {
+  display: inline-flex;
+}
+
 .approvals-badge {
   position: absolute;
   top: 2px;