Report test results in the CI run summary too

[jstayton]

Follow-up to #46, aligning it with the equivalent job in queryql:
TRUEPIC/queryql#96

Two changes.

The job already ran the whole suite but reported only coverage, so the test
results stayed in the job log. Node prints its own summary block immediately
before the coverage report, so surfacing it costs one more grep over output
we were already capturing.

More importantly, reporting now happens in its own step. The default shell is
bash -e, so the set -o pipefail added in #46 made a failing suite abort the
step before the summary block ran — meaning a red run got no summary at all,
losing both the results and the coverage report at the one moment they were
most worth reading. I checked with a deliberately failing test: bash -e with
a failing pipe exits 1 and never reaches the following block. Node emits both
its summary and the coverage report even when tests fail, so splitting the
report into its own step with if: ${{ !cancelled() }} is enough to keep them.

$RUNNER_TEMP/coverage.txt is renamed to tests.txt, since it now holds both.
AGENTS.md gets the same one-line correction.

Rendered summary:

## Tests

tests 37
suites 4
pass 37
fail 0
cancelled 0
skipped 0
todo 0
duration_ms 424.943458

## Test coverage

----------------------------------------------------------
file      | line % | branch % | funcs % | uncovered lines
----------------------------------------------------------
src       |        |          |         |
 error.js | 100.00 |   100.00 |  100.00 |
 main.js  | 100.00 |   100.00 |  100.00 |
----------------------------------------------------------
all files | 100.00 |   100.00 |  100.00 |
----------------------------------------------------------

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