feat: default crabcode to codex

[MrFlounder]

Why

Crabcode is moving to Codex as the default experience, and the repo's release metadata was inconsistent: the CLI version in src/crabcode was already 0.13.0 while release-please metadata still said 0.12.0.

What

• default missing-agent and blank-main behavior to Codex, while preserving legacy Claude inference for older configs that explicitly hardcoded a Claude main command
• make crab court user-facing behavior Codex-led: Codex judge, generic teammate Reviewer A, Codex Reviewer B, and a Codex requirement for court sessions
• update README, top-level docs, examples, and promptfoo defaults to present Codex/GPT-5 as the defaults; remove Claude hints from crab court, README, and the core docs surfaces
• align release metadata to 0.13.0 and add an Unreleased changelog note for the Codex-default change
• keep internal Claude compatibility paths intact; this PR does not remove Claude support repo-wide

How to Test

• run bash -n src/crabcode
• run git diff --check
• run cd plugins/promptfoo && npm run build
• verify rg -n "Claude|claude" README.md docs/index.html docs/llms.txt returns no matches
• check the crab court help text and session metadata to confirm a Codex judge, teammate Reviewer A, and Codex Reviewer B
• note: the root formatter step pnpm exec biome check --write . is not runnable in this repo because there is no root package.json or pnpm-workspace.yaml (ERR_PNPM_RECURSIVE_EXEC_NO_PACKAGE)