feat: add Codex second opinion to /office-hours (Phase 3.5)

New generateCodexSecondOpinion resolver that adds an opt-in cross-model
cold read between premise challenge and alternatives generation.
Codex independently reviews the session's problem statement, answers,
and premises without seeing Claude's reasoning.

Includes: temp file prompt assembly (shell injection safe), two mode-
specific prompt variants (startup/builder), cross-model synthesis,
premise revision check, and 7 unit tests.

office-hours/SKILL.md.tmpl

@@ -348,6 +348,10 @@ Use AskUserQuestion to confirm. If the user disagrees with a premise, revise und
 
 ---
 
+{{CODEX_SECOND_OPINION}}
+
+---
+
 ## Phase 4: Alternatives Generation (MANDATORY)
 
 Produce 2-3 distinct implementation approaches. This is NOT optional.
@@ -374,6 +378,7 @@ Rules:
 - One must be the **"minimal viable"** (fewest files, smallest diff, ships fastest).
 - One must be the **"ideal architecture"** (best long-term trajectory, most elegant).
 - One can be **creative/lateral** (unexpected approach, different framing of the problem).
+- If Codex proposed a prototype in Phase 3.5, consider using it as a starting point for the creative/lateral approach.
 
 **RECOMMENDATION:** Choose [X] because [one-line reason].
 
@@ -397,6 +402,7 @@ Track which of these signals appeared during the session:
 - Has **domain expertise** — knows this space from the inside
 - Showed **taste** — cared about getting the details right
 - Showed **agency** — actually building, not just planning
+- **Defended premise with reasoning** against cross-model challenge (kept original premise when Codex disagreed AND articulated specific reasoning for why — dismissal without reasoning does not count)
 
 Count the signals. You'll use this count in Phase 6 to determine which tier of closing message to use.
 
@@ -450,6 +456,9 @@ Supersedes: {prior filename — omit this line if first design on this branch}
 ## Premises
 {from Phase 3}
 
+## Cross-Model Perspective
+{If Codex ran in Phase 3.5: Codex's independent cold read — steelman, key insight, challenged premise, prototype suggestion. Verbatim or close paraphrase of what Codex said. If Codex did NOT run (skipped or unavailable): omit this section entirely — do not include it.}
+
 ## Approaches Considered
 ### Approach A: {name}
 {from Phase 4}
@@ -499,6 +508,9 @@ Supersedes: {prior filename — omit this line if first design on this branch}
 ## Premises
 {from Phase 3}
 
+## Cross-Model Perspective
+{If Codex ran in Phase 3.5: Codex's independent cold read — coolest version, key insight, existing tools, prototype suggestion. Verbatim or close paraphrase of what Codex said. If Codex did NOT run (skipped or unavailable): omit this section entirely — do not include it.}
+
 ## Approaches Considered
 ### Approach A: {name}
 {from Phase 4}