Add local runner tests

[nickbeau]

Detail: Test command construction, environment injection, port allocation, and lifecycle state transitions.

Milestone: Sprint 7: Local Process Runner

Source: docs/technical.md sprint plan.

[nickbeau]

I’m starting work on this now. I’ll inspect the local runner implementation and existing test structure, then add focused coverage for command construction, environment injection, port allocation, and lifecycle state transitions before running the relevant build/test commands.