docs: record the CI selection design and its plan

The binding rule table (17 rows x 3 answer columns), the measured effect per PR
shape, and the reasoning behind the parts that look surprising: why empty means
empty, why hw/mcu and lib are rules rather than full-matrix paths, why get_deps.py
is diffed as data, and which build system is the reference. The plan is the
task-by-task record of how it was built, kept as the origin trail.