fix: use a valid mtplx scheduler mode in the example preset

[outofcoffee]

Fixes spinloop serve failing when the mtplx example preset referenced a scheduler mode that mtplx has since removed.

Summary

• mtplx 2.11.1 removed the old serial/parallel/concurrent --scheduler-mode choices, so the shipped example preset (scheduler-mode = parallel) made spinloop serve exit with an invalid choice.
• Use ar_batch in the example preset and the two serve test fixtures that mirror it.
• Stop pinning an exact scheduler-mode list in docs/commands/serve.md and the example README; both now note that serve passes the value through unchecked and that mtplx serve --help lists the modes the installed build offers.

Implementation details

ar_batch was chosen because it is the mode behind mtplx's open_code_fair scheduling policy, its configuration for coding-agent (opencode) traffic, which is what this example serves. The docs no longer enumerate the valid modes: that pinned list is what broke when mtplx renamed its modes. The main specs never pinned the values, so they needed no change.