[agent builder] Document Elastic AI Agent is now mutable

[leemthompo]

Documents changes coming in elastic/kibana#256333

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

[github-actions]

Vale Linting Results

Summary: 3 suggestions found

💡 Suggestions (3)

File	Line	Rule	Message
explore-analyze/ai-features/agent-builder/builtin-agents-reference.md	23	Elastic.Clone	Use clone only when referring to cloning a GitHub repository or creating a copy that is linked to the original. Often confused with 'copy' and 'duplicate'.
explore-analyze/ai-features/agent-builder/builtin-agents-reference.md	75	Elastic.Clone	Use clone only when referring to cloning a GitHub repository or creating a copy that is linked to the original. Often confused with 'copy' and 'duplicate'.
explore-analyze/ai-features/agent-builder/custom-agents.md	110	Elastic.Clone	Use cloned only when referring to cloning a GitHub repository or creating a copy that is linked to the original. Often confused with 'copy' and 'duplicate'.

---

The Vale linter checks documentation changes against the Elastic Docs style guide.

To use Vale locally or report issues, refer to Elastic style guide for Vale.