feat(on): add generic WorkflowRunner interface

[darlanalves]

Motivation

• Provide a shared, strongly-typed contract for different runner implementations (e.g., shell, docker, node) so multiple packages can implement runners against a common interface.
• Allow consumers to specialize step, context, and output types while keeping a consistent runtime contract for executing workflow steps.

Description

• Added a new exported WorkflowRunner interface to packages/on/src/types.ts that is generic over TStep, TContext, and TOutput and defaults to StepDefinition, WorkflowContext, and StepOutput respectively.
• The interface defines the contract run(step, context): Promise<TOutput> modeling an asynchronous execution that returns an exit code, stdout/stderr and the executed command.
• The new interface is colocated with existing types (StepDefinition, WorkflowContext, StepOutput) so implementations can import and specialize the runner type consistently.

Testing

• Ran pnpm nx run on:build, which completed successfully.
• Ran pnpm nx run on:test, which executed but failed in this environment due to missing Docker (spawn docker ENOENT).
• Test failures occurred in src/cli.spec.ts with 3 failing tests, which is caused by the unavailable Docker runtime rather than the new type addition.

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Codex Task