docs: record the pipe-masking gate trap in the handoff

Review round 2 found no new defects; this is the one durable item it
asked to be carried further than a commit message.

`cmd | tail -2` returns TAIL's exit status, not `cmd`'s, in fish and
bash alike. So a gate chain of `cargo test ... | tail -2 && ... && echo
"ALL GATES CLEAN"` prints the clean line even when a suite failed, and
that is what happened while gating this lane: a `pmacs-gpu` failure was
summarized as clean. The point worth keeping is that this is not
carelessness a closer read would catch --- the verdict is structurally
absent from the summary the PR then cites. §5 now says to check
`$pipestatus[1]`, or better to redirect each gate to a file and read it
afterwards, which also preserves the full log that section already asks
for. Filed beside the skip-reports-`ok` lesson, which is the same
family: the thing that summarizes a gate must not be able to lose the
gate's verdict.

Also fixes the doc-comment splice in `tests/support/mod.rs`, where the
why-two-directories paragraph landed mid-sentence and left the
include-mechanics explanation stranded inside it. Cosmetic, and review
called it not worth a round on its own --- folded in here because the
file was being touched anyway.

docs/agent-handoff.md

@@ -1443,6 +1443,22 @@ round-trip cannot detect a discriminant shift.
 
 ## 5. Hard-won ops lessons
 
+- **A gate summary assembled through a pipe can report success over a
+  failure.** `cmd | tail -2` returns **`tail`'s** exit status, not
+  `cmd`'s — in `fish` and `bash` alike — so a chain of
+  `cargo test ... | tail -2 && cargo test ... | tail -2 && echo "ALL
+  GATES CLEAN"` prints the clean line even when a suite failed. This
+  is not carelessness that closer reading would catch: the failure is
+  **structurally invisible** in the summary the PR then cites. It
+  happened while gating the silent-skip lane, and a `pmacs-gpu`
+  failure was reported as clean.
+
+  Either check `$pipestatus[1]` in fish (`${PIPESTATUS[0]}` in bash),
+  or — better — redirect each gate to a file and read the file
+  afterwards, which also preserves the full log this section already
+  asks you to keep. Same family as the skip-reports-`ok` lesson below
+  and the double-invocation traps: **the thing that summarizes a gate
+  must not be able to lose the gate's verdict.**
 - **A test that skips on a missing precondition reports `ok`, and a gate log
   cannot tell that apart from a pass.** `vterm_stage3_acceptance::a37` — the
   only acceptance driving a real daemon, a real PTY and a real wgpu render

tests/support/mod.rs

@@ -1,7 +1,13 @@
 //! Shared test-support helpers.
 //!
 //! Included by `#[path = "support/mod.rs"] mod support;` rather than
-//! copied.
+//! copied. Files under `tests/` subdirectories are not compiled as
+//! their own test binaries, so this costs nothing — and
+//! `m6_8_multi_repl_acceptance.rs` previously carried a comment saying
+//! cross-test-binary sharing "would need a fixture crate", which is not
+//! so. A correct helper in one file and a degraded copy in another is
+//! this suite's most repeated defect shape; sharing removes the way it
+//! happens.
 //!
 //! **Why this is separate from `tests/common/`, which also exists.**
 //! `tests/common/mod.rs` re-exports `daemon` and `pty` — real daemon
@@ -11,13 +17,7 @@
 //! dependency-free half: helpers any test binary can take without
 //! taking a subsystem with them. Two directories is a cost worth
 //! naming rather than leaving to be rediscovered; if a third appears,
-//! consolidate instead of continuing the pattern. Files under `tests/` subdirectories are not compiled as
+//! consolidate instead of continuing the pattern.
-//! their own test binaries, so this costs nothing — and
-//! `m6_8_multi_repl_acceptance.rs` previously carried a comment saying
-//! cross-test-binary sharing "would need a fixture crate", which is not
-//! so. A correct helper in one file and a degraded copy in another is
-//! this suite's most repeated defect shape; sharing removes the way it
-//! happens.
 
 #![allow(dead_code)]