CLAUDE_CODE_SUBAGENT_MODEL Changed From an Override to a Default, So Agent Definitions Now Win

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2.1.251 changes `CLAUDE_CODE_SUBAGENT_MODEL` to set the default subagent model rather than override everything: an agent definition's `model:` frontmatter and an explicit per-spawn model now take precedence over it. Anyone who set that env var to force every subagent onto Haiku for cost control has silently lost that guarantee for agents that declare their own model. Check your `.claude/agents/*.md` frontmatter before assuming the env var still caps spend.