Move the Remaining Prose-Matched Subprocess Failures Onto the Status-Code Rule #3378

Context

The CODESTYLE.md "Operations and Failure Messages" subprocess-failure bullet, as revised by #3377 for #3367, classifies a subprocess failure by which command failed and the status code it reports, never by the prose of its message. A printed code is read by the notation that marks it, and a match finding none counts as an unexpected failure. A local review pass on #3377 and a sweep of the scripts found these sites still classifying by prose. The unknown flag retry in scripts/pr_review.py is tracked separately in #3365.

Sites

• spec/audit.py gh(), the ok404 branch: "HTTP 404" in r.stderr or "Not Found" in r.stderr. "Not Found" is prose, and a bare HTTP 404 substring is not the (HTTP 404) notation, so any message carrying either reads as absence rather than as an unexpected failure.
• scripts/skills_install.py, the claude plugin marketplace add and claude plugin install calls: each treats a non-zero exit as success when "already" appears in its stdout or stderr. That is classification by the prose of the message, so a version that rewords it, or another failure that happens to say "already", is misread.

Remedy

Move each onto the rule in a change of its own: the status code the tool reports, a printed code read by its notation, or an operation of its own for the expected failure, such as a read that says whether the marketplace or plugin is already present.