fix(pi): run plannotator_submit_plan sequentially so it never reads a stale plan (#1623)

Pi runs one assistant message's tool calls in parallel by default. Its
write/edit tools apply changes through an async per-file mutation queue,
while plannotator_submit_plan reads the plan file synchronously as soon as
it starts, so an "edit plan + submit plan" batch could review (and save to
history) the pre-edit plan. Declaring executionMode "sequential" makes pi
run the whole batch one call at a time, in order.

Closes #1622