Custom process: command Steps and optional Steps #1382

Parent

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

What to build

A Step can run command instead of a skill: a command or script run in its worktree, deterministically and without tokens. Its stdout and stderr together become the Step's result, passed on like a final message. Environment variables give the issue URL (when there is one), the results folder and the Step's own result file. A non-zero exit fails the Step. The first failed required Step stops the Process. A Step marked optional = true has its failure recorded, and the Process carries on.

Acceptance criteria

• A command Step runs with its worktree as the working directory, and a later session Step's prompt names the path of its output.
• A command Step that exits non-zero fails the Process, and later Steps don't run. The Command log names the Step and its exit status.
• An optional Step that fails is recorded as failed in the Command log, and the Process carries on to success.
• The environment variables are set, and are documented in docs/guide.md.
• Stop interrupts a running command Step as it does a session.