New CLAUDE_CODE_SUBAGENT_MODEL_FORCE env var forces subagent model

A new environment variable, CLAUDE_CODE_SUBAGENT_MODEL_FORCE, forces the Task tool to ignore a per-call model override for subagents/workflow agents. When set, the model parameter is stripped from the Task tool schema (n.omit({ model: !0 })), and workflow agent model requests are dropped with a logged message.

The in-tool Workflow authoring reference also checks CLAUDE_CODE_SUBAGENT_MODEL_FORCE: when set, it omits the model field from the documented agent() opts and drops the guidance text about setting model overrides in phase() entries and agent() calls, since a forced subagent model makes that option moot.