feat(workflows): wire TaskUnit/Orchestrator activities, add k8s deploy manifests

Implements real activity-calling logic in OrchestratorWorkflow and
TaskUnitWorkflow (previously stubs), adds GitDiffActivity, and expands
PlanningActivity's I/O to carry repo path and prior task results.

Adds k8s/ deployment manifests (worker Deployment, orchestrator Job,
Kustomize base) for the poimen-workflows Temporal worker, using a
dedicated Kubernetes namespace `poimen` and Temporal namespace
`poimen-harness` rather than sharing the Temporal server's own
`temporal`/`production` namespaces.

statemachine/types.go

@@ -120,3 +120,9 @@ func NewActivityTuning() ActivityTuning {
 		PiRetry:                NewPiRetryPolicy(),
 	}
 }
+
+// PromptUpdate represents an update to a role prompt.
+type PromptUpdate struct {
+	Role string
+	Spec PromptSpec
+}