fix(pm): make the stamp contract quotable — Markdown code is a quotation, rendered as written

`post-stamped` substituted the act-clock token wherever it appeared, a passage
QUOTING the token included, so a seat could not quote the tool's own diagnostic
through the tool. Measured three times inside one hour on live artefacts: a
false quotation published on #14251 (an inline code span inside a blockquote),
the same hit in the sentence describing it and again in the comment reporting
that, plus a dispatch claim refused `[quoted-not-a-stamp]` for spelling the
contract's own documentation placeholder.

The remedy is structural, not a third spelling and not a flag: inside Markdown
code — a fenced block or a backtick span — this tool renders TEXT. Substitution,
the opener scan and the quoted-stamp validation all stand down there; a reader of
the stored artefact sees the rule without knowing the tool exists.

It suppresses nothing that judges a stamp a human typed. The positional scan
reads every line, code included; the MIXED trigger stays deliberately
quote-unaware; and `maskQuotedStamps` blanks a quoted-stamp token only where it
IS a token, so no stamp can hide from the contract behind backticks.

The status line now reports how many openers were left verbatim beside how many
were substituted, so the count is no longer the only signal an author has to
compare against intent.

Self-test: 257 cases across 11 batteries -> 308 across 12.

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

Author: claude