docs: retract a false claim in the handoff — the runner does NOT exit 0 on failure

[ZacxDev]

#308 asserted run-tests.sh "can exit 0 while printing RESULT: FAIL / collected=0". It doesn't, and it can't.

if [ "$fail" -ne 0 ]; then
  echo "RESULT: FAIL"
else
  echo "RESULT: PASS"
fi
exit "$fail"

Measured on 13bc8bd: a run without pytest importable prints 17 × "could not parse pytest's summary", RESULT: FAIL, exit 1 (2 in a second probe). The original report most likely read a status through a pipeline — the rc=$? trap RULES.md already names.

The real defect (fixed in #310) is narrower: REQUIRED_TOOLS checks binaries via command -v, but pytest is a module, so a missing pytest produced a diagnostic blaming pytest's output format rather than a clean precondition failure. Bad diagnostics, not a false green.

I relayed this from a subagent report and repeated it to the operator twice without testing it — the same class of error as the four corrections the doc itself records, except this one reached a merged document first. Kept as strikethrough rather than deleted, since the doc's whole purpose is recording what was believed and then disproved.

Also closes three items: #309 (529 ungated node tests, not 508 — collector/browser-ext/tests was ungated too; gate 468 → 997), #311 (deadman, including the verified "cannot tell ≠ healthy" property), and session-create's cause (session.created is a bus event type, not a hook name — 2,736 of 2,799 tool-call rows carry session='').

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