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(superseded by the defineProperty accessor this PR adds).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/session.ts

@@ -125,7 +125,7 @@ const CTRL_L_PATTERN = /\x0c/g;
 const NEWLINE_SPLIT_PATTERN = /\r?\n/;
 
 /** True for external-CLI run modes (non-Claude) that use their own TUI and output format. */
-function isExternalCliMode(mode: SessionMode): boolean {
+export function isExternalCliMode(mode: SessionMode): boolean {
   return mode === 'opencode' || mode === 'codex';
 }