office-hours: Phase 3.5 should fall back to Claude subagent when Codex is unavailable

[shohu]

Problem

/office-hours Phase 3.5 (Cross-Model Second Opinion) silently skips entirely when codex CLI is not installed:

If CODEX_NOT_AVAILABLE: skip Phase 3.5 entirely — no message, no AskUserQuestion. Proceed directly to Phase 4.

Additionally, line 662 explicitly prohibits fallback:

On any error, proceed to Phase 4 — do NOT fall back to a Claude subagent (this is brainstorming, not adversarial review).

This is inconsistent with /plan-eng-review and /ship, which both fall back to a Claude adversarial subagent when Codex is unavailable:

• plan-eng-review line 707: "If CODEX_NOT_AVAILABLE (or Codex errored): Dispatch via the Agent tool."
• ship line 1176: "Claude adversarial subagent (fallback when Codex unavailable or errored)"

Impact

Users without codex installed never get a second opinion during office-hours, even though the infrastructure (Agent tool subagent) is available and already proven in other skills.

Proposed Fix

Apply the same pattern as plan-eng-review:

1. Always show the AskUserQuestion (regardless of codex availability)
2. If codex is available, use it
3. If codex is unavailable or errors, fall back to Claude subagent via Agent tool
4. Present under SECOND OPINION (Claude subagent): header