refactor(workflows): extract invokeStep from settle

[TheGreatAxios]

Extracts the per-step dispatch block from settle into a private invokeStep method with zero behavior change.

settle keeps the loop skeleton, frame exhaustion and pop, parent advance, the undefined-step guard, and a single call site. invokeStep returns true when it lands on an executable step and false when the loop should continue; throws propagate unchanged.

Six behavior pins appended to tests/unit/workflows-runtime.test.ts. Baseline event log captured on the unmodified tree and diffed byte-identical after the extract.

Fixes CL-8289

[linear-code]

CL-8289

[TheGreatAxios]

Closing without merge: the step-runtime/workflow cutover was removed from 0.3.33 and moved to the v0.4.2 runtime tranche. The current PR is also not the runSubAgentInner → invokeStep seam described by the Linear issue, so it should not be carried forward as the implementation. No code was merged.