feat(commands): add santa-loop adversarial review command (#1052)

* feat(commands): add santa-loop adversarial review command

Adds /santa-loop, a convergence loop command built on the santa-method
skill. Two independent reviewers (Claude Opus + external model) must
both return NICE before code ships. Supports Codex CLI (GPT-5.4),
Gemini CLI (3.1 Pro), or Claude-only fallback. Fixes are committed
per round and the loop repeats until convergence or escalation.

* fix: address all PR review findings for santa-loop command

- Add YAML frontmatter with description (coderabbit)
- Add Purpose, Usage, Output sections per CONTRIBUTING.md template (coderabbit)
- Fix literal <prompt> placeholder in Gemini CLI invocation (greptile P1)
- Use mktemp for unique temp file instead of fixed /tmp path (greptile P1, cubic P1)
- Use --sandbox read-only instead of --full-auto to prevent repo mutation (cubic P1)
- Use git push -u origin HEAD instead of bare git push (greptile P2, cubic P1)
- Clarify verdict protocol: reviewers return PASS/FAIL, gate maps to NICE/NAUGHTY (greptile P2, coderabbit)
- Specify parallel execution mechanism via Agent tool (coderabbit nitpick)
- Add escalation format for max-iterations case (coderabbit nitpick)
- Fix model IDs: gpt-5.4 for Codex, gemini-2.5-pro for Gemini