Using your example data:

create table #Assigned
(
    Agent varchar(20),
    task varchar(20),
    taskDate date
)

create table #Stalking
(
    Agent varchar(20),
    stalkDate date
)

insert into #Assigned values
('dbl07',    'Stalking','2019-12-01'),
('dbl07',    'patrol',  '2020-06-01'),
('dbl07',    'travel',  '2020-06-24'),
('dbl07',    'Stalking','2021-10-01'),
('dbl07',    'Stalking','2021-12-01'),
('dbl07',    'Stalking','2022-12-24'),
('dbl07',    'travel',  '2022-12-01'),
('mrBourne', 'train',   '2021-06-06'),
('mrBourne', 'Stalking','2022-06-06'),
('mrSmart',  'Stalking','2018-01-01'),
('mrSmart',  'idle',    '2021-06-06'),
('mrSmart',  'Stalking','2021-01-01'),
('mrSmart',  'Stalking','2022-06-06'),
('mrEnglish','lurk',    '2022-06-06'),
('mrEnglish','jest',    '2022-06-06')

insert into #Stalking values
('dbl07',    '2020-01-31'),
('dbl07',    '2022-12-31'),
('mrBourne', '2022-06-30'),
('mrSmart',  '2022-06-30'),
('mrEnglish','2022-06-30'),
('mrPowers', '2022-06-06')

You can get do something like:

select distinct
    ag.Agent,
    mm.minDate,
    mm.maxDate,
    tot.numRecords,
    isnull(tot.numStalking,0) as numStalking,
    case when tot.numRecords = 0 or isnull(tot.numStalking,0) = 0
        then '0.00%'
        else format(convert(decimal(10,2),isnull(tot.numStalking,0)) / convert(decimal(10,2),tot.numRecords),'P')
    end as prcnt
from 
(select distinct Agent from #Stalking) ag
left join #Assigned a on ag.Agent = a.Agent
outer apply
(
    select 
        min(stalkDate) as minDate,
        max(stalkDate) as maxDate
    from #Stalking s
    where s.Agent = a.Agent
) mm
outer apply
(
    select 
        count(*) as numRecords,
        sum(case when ass.task = 'Stalking' then 1 else 0 end) as numStalking
    from #Assigned ass
    where ass.taskDate >= mm.minDate
    and ass.taskDate <= mm.maxDate
    and a.Agent = ass.Agent
) tot

which gives the output:

This actually differs from your stated expected output, but looking at your example data, I believe 50% is the correct percentage for dbl07 as they have 6 records in Assigned which fall into their range of dates in Stalking , three of which have the task value of "Stalking" .

There might be a more elegant way to do this without OUTER APPLY (perhaps using Window Functions), but this is just the method which came to mind.