fix(codex): review fixes — envelope handling, mode guards, UI parity

Blocker: runCodex read raw response shapes, but the global
preSerialization hook (server.ts) wraps every payload in the
{ success, data } envelope — status.available was always undefined, so
the UI unconditionally printed "Codex CLI not found" and could never
start a session; the created session was also never auto-selected
(data.sessionId vs data.data.sessionId). Fixed both reads to match
runOpenCode, and updated the test mocks to the real wire shape (plus a
selectSession assertion) so envelope drift fails the test.

Guard parity: export isExternalCliMode() from session.ts and use it in
the ralph-config guard, all three respawn guards, and the six restore/
setup guards in server.ts that previously only excluded 'opencode' —
codex sessions could otherwise get a Ralph tracker or respawn
controller attached (idle detection is Claude-specific and output-
silence respawn cycling would misfire on a quiet codex TUI).

UI parity: cx tab badge, "Kill Tmux & Codex" dialog title, and the
missing CSS (.run-mode-dot.codex, .tab-mode.codex, .mode-codex button
colors — purple) so the Codex menu dot is no longer invisible. Removed
the dead object-literal runMode getter that Object.assign flattens
(superseded by the defineProperty accessor this PR adds).

Verified end-to-end on an isolated instance with a stub codex binary:
10/10 Playwright checks (menu/dot/label/button styling, session
created + auto-selected, cx badge, TUI output streamed, ralph+respawn
guards reject codex) and --dangerously-bypass-approvals-and-sandbox
+ --model observed on the spawned command line.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/public/app.js

@@ -2572,7 +2572,7 @@ class CodemanApp {
           <span class="tab-status ${status}" aria-hidden="true"></span>
           <span class="tab-info">
             <span class="tab-name-row">
-              ${mode === 'shell' ? '<span class="tab-mode shell" aria-hidden="true">sh</span>' : mode === 'opencode' ? '<span class="tab-mode opencode" aria-hidden="true">oc</span>' : ''}
+              ${mode === 'shell' ? '<span class="tab-mode shell" aria-hidden="true">sh</span>' : mode === 'opencode' ? '<span class="tab-mode opencode" aria-hidden="true">oc</span>' : mode === 'codex' ? '<span class="tab-mode codex" aria-hidden="true">cx</span>' : ''}
               <span class="tab-name" data-session-id="${id}">${(() => { const p = parseSessionPrefix(name); return p && p.suffix ? '<span class="tab-prefix">' + escapeHtml(p.prefix) + '</span><span class="tab-suffix">: ' + escapeHtml(p.suffix) + '</span>' : escapeHtml(name); })()}</span>
               <span class="tab-detached-badge" aria-hidden="true">detached</span>
             </span>
@@ -3374,7 +3374,9 @@ class CodemanApp {
     if (killTitle) {
       killTitle.textContent = session.mode === 'opencode'
         ? 'Kill Tmux & OpenCode'
-        : 'Kill Tmux & Claude Code';
+        : session.mode === 'codex'
+          ? 'Kill Tmux & Codex'
+          : 'Kill Tmux & Claude Code';
     }
 
     document.getElementById('closeConfirmModal').classList.add('active');