fix(pm): a retracted claim leaves the governing-claim pool

The pool was every comment matching CLAIM_COMMENT_MARKER, ranked by
recency. A retraction carries no `Claim:` line, so it was never in the
pool and could not remove the claim it retracts: on #18373 a claim its
own author withdrew 84 seconds later kept governing, naming a branch
origin does not have, while the seat actually working the card read
SUPERSEDED.

Membership is now resolved before recency. A claim leaves the pool when
a later comment by the same author retracts it, read through the two
channels the protocol already writes: the `Release:` directive, and a
prose line that opens with the retraction act and names the claim by
comment id. A retracted claim is listed RETRACTED with the retracting
comment id, never SUPERSEDED and never dropped.

Claude-Session: https://claude.ai/code/session_01Gqi43smmqjJ5sUrhfoPeKu
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude