docs(cli): document env-var apiKey support for custom models

[factory-arman]

Summary

• document ${VAR_NAME} support for customModels[].apiKey in settings.json/settings.local.json
• update BYOK examples to show env var usage instead of hardcoded API keys
• clarify that env expansion does not apply to legacy ~/.factory/config.json

[factory-droid]

Droid finished @factory-arman's task —— View job

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• Fetched PR metadata + diff + file patches
• Reviewing for any P0/P1 issues and reconciling prior bot comments/threads

[factory-droid]

Changes look correct: they consistently document ${VAR_NAME} env-var expansion for apiKey in settings.json/settings.local.json, and clarify that legacy config.json does not get env expansion. No P0/P1 issues found.