Add engine.command field to use custom CLI executable

[Copilot]

Adds engine.command field to specify a custom executable path, skipping standard installation steps and using the custom command to invoke the agentic engine.

Changes

• EngineConfig: Added Command field to store custom executable path
• Schema: Added command property to engine configuration schema with description
• Installation Logic: Modified GetInstallationSteps() for Copilot, Claude, and Codex engines to return empty when command is set
• Execution Logic: Modified execution steps for all engines to use the custom command instead of the default command names
• Tests: Added parsing and installation-skip tests for all engine types

Usage

engine:
  id: copilot
  command: /usr/local/bin/custom-copilot

When command is specified:

• Skips secret validation, Node.js setup, and CLI installation
• Custom command replaces the default engine command (copilot → custom command value, claude → custom command value, codex → custom command value)
• All CLI arguments remain unchanged and are passed to the custom command

Useful for self-hosted runners with pre-installed CLIs at custom paths or custom CLI wrappers.

Original prompt

Add a field to engine configuration frontmatter to provide a custom executable path.

engine:
...
command: "..."

When this flag is enabled, do not emit installation step.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[pelikhan]

@copilot the arguments to the cli are unchanged

[pelikhan]

@copilot "command" should replace the default command used to invoke the agentic engine

copilot --> command_value
claude --> command_value

[Copilot]

@copilot "command" should replace the default command used to invoke the agentic engine

copilot --> command_value...

Updated in commit 5308f5c. The command field now replaces the default command in execution:

• copilot → custom command value
• claude → custom command value
• codex → custom command value

All CLI arguments remain unchanged and are passed to the custom command.