hook: pre-compact.sh and session-end.sh

Part 3 · EACH TOOL · Slide 51 of 60

pre-compact (PreCompact, 10 s): one line — bank before the
context shrinks.
session-end (SessionEnd, 20 s, reads no stdin): the crash
cushion — one COORD line if the session did not close through
/sessionend, then the volume roll: COORD.md seals as
COORD-<NNN>.md at ~500 lines, COORD-AGENTS.md at 1000, copy
synced before the original is touched.