docs(agents): say which runner the EXIT-trap failure mode needs - #3779

§3 bans test level EXIT traps because such trap replaces the handler bats binary installs, so a failing test prints no TAP line at all and grep for not ok reads a green suite. That is true, and it does not happen in CI.

hack/cozytest.sh runs each @test inside explicit subshell and reads the status out of band on the next line, so a trap in a test body fires when that subshell exits and takes nothing with it. Real bats binary is not called anywhere in this tree, make bats-unit-tests and every recipe in packages/core/testing/Makefile go through cozytest. Disappearing not ok needs someone running bats by hand on a file.

Worth writing down because the rule already cost four PRs and three successive mechanisms, and the missing note reads as unpaid debt, 68 declared traps that a reader assumes CI is one refactor away from needing. The bullet also records that two of the covered files resist a mechanical conversion, one body cd's into its scratch dir and never returns so an end of body cleanup would delete its own cwd, another has its last command deciding the verdict. Next person to try that should not find it halfway through.

Summary by CodeRabbit

• Documentation
  • Clarified that the end-to-end test runner preserves failing test results when executing BATS tests in subshells.
  • Explained that disappearing failure results can occur only when invoking the BATS binary manually.
  • Expanded guidance on cleanup behavior, trap-count checks, sourced handlers, and runner-specific cases where removing exit traps may change results.