docs: document the plan-root pin and correct the recitation token figure

The skill text described per tool call re-injection as "the +68% token tax
measured in the v2.21 eval". That eval measured total tokens for a whole task
with the skill against the same task without it, and its own write-up
attributes the difference to writing three structured files instead of one or
two. It never isolated recitation. Raised while answering #210; the wording
now states the measured per call figure instead of borrowing an unrelated
number.

PWF_PLAN_ROOT existed only in code comments and a refusal message, so the
escape hatch a user is told to reach for was undocumented. The skill now
explains the shared parent case and the ambiguity refusal, and the README
gains an environment variable table covering all seven knobs, several of which
had never been written down anywhere, including PWF_SESSION_ID.

README test count refreshed.