feat(pm): check-prior-rulings reads the card's own thread as a corpus half

The `Prior rulings read:` line answered for three tree corpora (docs/adr,
AGENTS.md, packages/spec/src) and not for the cheapest ruling there is: the
one already written on the card being checked. Measured on one card, a
director-seat ruling was followed four days later by a retriage that moved
the card back into the decision box without citing it, the maintainer ruled
again with a different prescription, and a dispatch round ended with zero
diff between the two. Run on that card, this file could not have said so.

So the card's own thread is the fourth corpus. A ruling comment is
recognised by the SHAPE of its first content line — the charter's
`Ruling: batch #n item k …`, the pre-charter `## Ruling recorded — …`
heading, and a director `Ruling addendum` — measured over 1,100 comments in
two windows of the live board; not by author (the director seat wrote as
three logins in five days) and not by a quoted 「同意」 (63 of 500 recent
comments carry one). Two measured near-misses are pinned OUT and listed
uncounted. The line gains a `thread:` clause that spells `none` (read, no
ruling), `not read (no --card)` (a --terms-only run) and the whole-line
`unresolved` (a thread that could not be read, exit 3) apart.

The read is one paged GET walked by page number to a short page, bounded,
with pages, bytes and wall time measured and printed: the incident card is
one page and ~0.35 s; the longest thread on the board (962 comments) is
ten pages and ~4.4 s. Every `--card` run rides the file's existing proxy
re-exec. The self-test gains the thread fixtures (shapes, near-misses, an
offline paged board with a refusal, a bound and a network failure): 99 → 144
cases.

Claude-Session: https://claude.ai/code/session_01BTeBejoPUvRHN8WdAJC6oF
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude