spec v2: apply codex critique, add codex-reviewer subagent

- separate DB models from API types (no more "one struct rules all")
- drop utoipa, drop channel membership, drop model from users
- add seq ordering, soft delete, hashed tokens, same-channel reply constraint
- WS auth via first message instead of query param
- reorder stories to vertical slice (conversation model first, deploy early)
- add codex-reviewer subagent for parallel GPT-5.4 reviews
- update critic + ax skills to use codex-reviewer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/skills/critic/SKILL.md

@@ -56,6 +56,22 @@ Optional prose narrative follows after a blank line.
 - For research claims, demand evidence or explicit acknowledgment of speculation.
 - For vibecoded implementations, focus on correctness and security over style.
 
+## Parallel Codex Review
+
+On every critic invocation, **immediately** spawn the `codex-reviewer` subagent in the background before starting your own analysis:
+
+```
+Agent(subagent_type="codex-reviewer", run_in_background=true,
+  prompt="Critique: $ARGUMENTS. Read all relevant files. What will break? What's missing? What's over-engineered?")
+```
+
+Continue your own critique without waiting. When the codex-reviewer returns, integrate its findings into your verdict:
+- Codex issues you missed → add to `breakpoints`
+- Codex agrees with you → note in `survives` as independent confirmation
+- Codex disagrees → address in prose narrative
+
+The final verdict is yours — codex is a second opinion, not an override.
+
 ## Research-specific checks
 
 When critiquing RL transfer hypotheses or experimental design: