Record the lessons of #623 through #641 in AGENTS.md (#645)

Five lessons, each named for the PR and the input that showed it:
re-take a recorded negative control after the last commit that adds
tests (three went stale in a row); a mutation that moves nothing shows
code unexercised, not dead (#638); a claim that a change restores the
released reading is measured over the moved population, and a
population size is re-counted, never copied (#625); a group-side
predictor of the family comma's given part via `tail_reading`
disagrees with the walk on three shapes, an accepted gap (#623, #637);
and the title-particles go in every head list in both spellings (#637).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: derek73