Create an mcx optimization

Choose the mechanism from the work, not from terminology the user happens to know.

Decide

1. Run mcx list so you do not duplicate an existing chain.
2. Use a matching registered chain when one exists.
3. For two or more related MCP calls, a fan-out, join, pagination loop, bulk mutation, or aggregation, follow references/chain.md.
4. For one MCP tool whose result is dominated by unused fields, follow references/filter.md.
5. When both signals appear, use an ad-hoc chain first. Its intermediate MCP payloads remain inside the sandbox, so filtering those internal calls is unnecessary unless the same tool is also used directly elsewhere.
6. Create nothing for a single compact isolated call.

Persistence boundary

Default to ad-hoc source executed with mcx run <args-json> <language> and a heredoc. Do not create a durable chain. When the user explicitly asks to save, register, name, or reuse the workflow, invoke /mcx:save after proving the ephemeral run.

Keep the final output to the smallest digest that answers the task. Never emit raw intermediate MCP payloads.