Custom process: per-Step Harness, Model and Effort #1381

Parent

#877 (Spec: Custom processes). Design record: docs/grill-with-docs/877-composable-by-file.md, ADR-0024.

What to build

A Step can name its own harness, model and effort. A Step that leaves one out uses the Command's, chosen as today: the command, then the User config, then the Harness's default. Every distinct Harness a Process uses gets the usual preflight before the first session, and one that can't start fails the Command before any session, with no fallback (a family you asked for is never silently replaced). The Security review already runs on a different Harness from the rest of its Command, so reuse that mechanism.

Acceptance criteria

• A Process with Steps on Claude, Codex and agy calls each fake Harness with the Model and Effort that Step asked for.
• A Step that names nothing uses the Command's Harness, Model and Effort, including --harness on the command line.
• An unknown Harness is a refusal naming the rule and line.
• A Harness that can't start fails the Command before the first session, naming the Step and the Harness. No session runs.
• thirdshift process check prints each Step's Harness, Model and Effort and where each came from (Step, command, User config, default), plus whether each Harness starts.
• GLOSSARY.md's Harness, Model and Effort entries change from "one per Command" to "one per Step, defaulting to the Command's". The Security review's exception stays.
• docs/guide.md's Custom processes section covers it.