Guard plan delegation ordering

[asim]

Summary

• Refuse delegate tool calls while earlier task-creation plan steps are still unfinished, preventing notification side effects from running before required launch tasks.
• Add deterministic coverage for the plan/delegate ordering guard.

Closes #3991
Closes #4002

Testing

• go build ./...
• go test ./agent -run TestPlanWrapBlocksDelegationUntilPriorPlanStepsFinish -count=1
• go test ./agent -run 'TestPlanWrapBlocksDelegationUntilPriorPlanStepsFinish|TestAgentProviderConformanceMatrix/fake' -count=1\n- go test ./internal/harness/plan-delegate -run 'TestZeroToHeroContract|TestPlanDelegateRetriesAfterUnknownDelegateTool' -count=1\n- go test ./... (fails in environment: loopback targets forbidden for grpc tests; mdns registry isolation flake)\n- golangci-lint run ./...