protect_first_n keeps the first N non-system messages verbatim through
compaction so the original task framing survives. But it was applied on
EVERY compression pass: the same early user turns were re-copied into each
child session and never summarized away, so across a long, repeatedly-
compressed session those old messages became immortal and grew the
protected head unboundedly (#11996, P1).
Decay it: protect_first_n applies on the FIRST compaction only. Once the
session has been compressed at least once (compression_count >= 1, or a
handoff summary already exists), the early turns are captured in the
summary, so _effective_protect_first_n() returns 0 and only the system
prompt stays protected. The decay is read at compress_start computation
time, before compression_count/_previous_summary are mutated at the end of
compress(), so the first pass still protects correctly.
Co-authored-by: truenorth-lj <liliangjya@gmail.com>
Co-authored-by: davidvv <david.vv@icloud.com>
