2.3.1 — third context notice at 1.5x, graded by rung

Middle rung at 1.5x the threshold (180k by default) joins 120k and 240k,
cutting worst-case silence between notices from 120k to 60k. The ladder is
multipliers, so TUNE_CONTEXT_REMIND_TOKENS scales all three.

Each rung is louder than the last: wait for a natural break, raise it at the
next pause anyway, say it this turn. Fixes the second notice calling itself
'second and final', which a three-rung ladder makes false.

Docs describing the reminder as firing once (README, tune-context skill) —
wrong since 2.1.0, corrected here.