* test(#3962): shipped TDD-Audit trailer token must round-trip through git
Behavioral coverage: extract the trailers:key token from ship.md and
prove a real git commit carrying that trailer reads back via
%(trailers:key=token,valueonly). gate_status contains an underscore,
which git's trailer machinery cannot tokenize, so the audit read was
structurally empty.
* fix(#3962): rename the TDD-Audit trailer token to gate-status
Underscore is not a valid git trailer token character, so
%(trailers:key=gate_status,...) could never match. Renames the token
at the read, the documented aggregate write, and the section's
prose/table header. Self-suppression semantics (#2431) unchanged.
* test(#3962): compare outcome against the seam's literal, fix doc token
Review follow-ups: the round-trip test compared outcome to 'EXITED'
but the process seam emits 'exited'; and the trailer rename is
propagated to docs/ship-pr-body-sections.md's read/write examples.
* chore(#3962): backfill changeset pr number
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Co-authored-by: sim <sim@local>
7960374d15