feat(codex): add Codex (OpenAI CLI) run-mode foundation

Add Codex as a first-class session mode alongside Claude/Shell/OpenCode.

- codex-cli-resolver: locate the `codex` binary and augment PATH (mirrors the
  OpenCode resolver)
- SessionMode 'codex' + CodexConfig (model, resumeSessionId, dangerouslyBypass,
  renderMode); persisted in SessionState and threaded through CreateSession/
  RespawnPane options
- schema validation: CodexConfigSchema, CODEX_ env-var prefix allowlist, mode
  enums on create/quick-start, codexDangerouslyBypassApprovals setting
- tmux launch: buildCodexCommand, setenv for OPENAI_API_KEY/CODEX_* (keeps
  secrets out of ps), truecolor COLORTERM, codex PATH resolution
- session + routes: availability check (clear install hint), config passthrough,
  tmux-required guard; Codex skips Claude-only parsers (Ralph/respawn/token)
- run-mode UI: "Run CX" selector option + dedicated Codex CLI settings tab with
  the bypass-approvals toggle; GET /api/codex/status

Scope: foundation only. Codex terminal redraw handling and xterm snapshot/replay
are intentionally excluded and tracked separately.

Verification: tsc --noEmit, eslint, prettier --check, check:frontend-syntax all
clean; full test:ci suite green (2712 passed, 0 failed); server boot smoke OK.

Co-Authored-By: Saqeb Akhter <saqeb.akhter@gmail.com>

src/mux-interface.ts

@@ -14,6 +14,7 @@ import type {
   ClaudeMode,
   SessionMode,
   OpenCodeConfig,
+  CodexConfig,
   EffortLevel,
 } from './types.js';
 
@@ -62,6 +63,7 @@ export interface CreateSessionOptions {
   claudeMode?: ClaudeMode;
   allowedTools?: string;
   openCodeConfig?: OpenCodeConfig;
+  codexConfig?: CodexConfig;
   /** When restoring after reboot, resume a previous Claude conversation by its session ID */
   resumeSessionId?: string;
   /** Extra env vars exported before launching the CLI (e.g., CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS). Ephemeral — not written to disk. */
@@ -80,6 +82,7 @@ export interface RespawnPaneOptions {
   claudeMode?: ClaudeMode;
   allowedTools?: string;
   openCodeConfig?: OpenCodeConfig;
+  codexConfig?: CodexConfig;
   /** Resume a previous Claude conversation when respawning */
   resumeSessionId?: string;
   /** Extra env vars exported before launching the CLI (preserved across respawns). */