fix: use CODEX_HOME instead of CODEX_CONFIG_DIR for per-group session isolation

CODEX_CONFIG_DIR is not recognized by Codex CLI. CODEX_HOME is the correct
env var that controls the root directory for sessions, state, and config.
Without this, all groups sharing the same workDir would have their sessions
collide, causing resume --last to pick up wrong group's session.

src/container-runner.ts

@@ -233,7 +233,7 @@ function prepareGroupEnvironment(
         fs.copyFileSync(src, dst);
       }
     }
-    env.CODEX_CONFIG_DIR = sessionCodexDir;
+    env.CODEX_HOME = sessionCodexDir;
   } else {
     // Claude Code — pass real credentials directly
     if (envVars.ANTHROPIC_API_KEY || process.env.ANTHROPIC_API_KEY) {