fix(codex): respect CODEX_HOME when reading auth for usage

[AndrewSB]

Problem

Codex thread execution correctly respected CODEX_HOME via resolveCodexHome() in list-commands.ts (process.env.CODEX_HOME?.trim() || path.join(homeDir, ".codex")), but codex-auth.ts (used for bb settings usage) hardcoded ~/.codex/auth.json.

This caused a split state when using a second account via CODEX_HOME:

• Threads: correctly used second account at $CODEX_HOME (e.g. /root/.codex-second -> asbreckenridge+automation@gmail.com) because host-daemon inherited env from systemd drop-in
• Usage UI: still showed main account from ~/.codex/auth.json (asbreckenridge@gmail.com / Team 64%)

Fix

Extract resolveCodexHome() helper in codex-auth.ts that checks process.env.CODEX_HOME first, matching existing logic in list-commands.ts and archive-codex-tmp-bb-sessions.ts.

function resolveCodexHome(): string {
  return process.env.CODEX_HOME?.trim() || path.join(os.homedir(), ".codex");
}
function codexAuthPath(): string {
  return path.join(resolveCodexHome(), CODEX_AUTH_RELATIVE_PATH[1]);
}

Repro

1. Have two Codex auths: ~/.codex and ~/.codex-second with different accounts
2. Add systemd override /root/.config/systemd/user/bb-server.service.d/codex-home.conf:

   [Service]
   Environment=CODEX_HOME=/root/.codex-second

3. systemctl --user restart bb-server
4. Spawn Codex thread: shows second account ✅
5. bb settings usage --json: shows first account ❌ (before fix), second after fix ✅

Verification

After patch + restart:

bb settings usage --json now returns:

{
  "codex": {
    "accountEmail": "asbreckenridge+automation@gmail.com",
    "planLabel": "Team",
    "usedPercent": 0
  }
}

Matches actual thread auth.