free-ci-health: a split shard's overrun compares wall time with its prediction over jobs (GST-1653) (#4085)

shardOverruns compared a shard's wall-clock elapsedMs with predictedMs, the
shard's serial seed sum. free-tests.yml plans every shard at --shard-jobs 4,
so the 1.5x warning only fired after about a 6x slowdown. The prediction is
now divided by the plan shard's jobs, ciHealthSummary carries jobs through,
and the warning names the parallel job count. k=1 output is unchanged.

Rewritten on main from #4051, which fixes #3851.

Co-authored-by: Indra Prasetya <indraoverflow@users.noreply.github.com>
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

docs/TESTING_INTERNALS.md

@@ -1594,7 +1594,9 @@ refresh to a maintainer.
 
 - `--ci-plan` warns in the job summary, with the refresh command, when more than 5
   files are unseeded; it never fails a PR. `--ci-verify` lists shards that ran
-  longer than 1.5× their prediction and at least 30 seconds over.
+  longer than 1.5× their prediction and at least 30 seconds over. A shard split
+  by `--shard-jobs k` runs its k sub-shards in parallel, so its prediction is the
+  serial seed sum divided by k.
 - The weekly test-health run fails its enforced `unseeded-free-files` check
   ("stale seed: N unseeded file(s) > 5") when more than 5 free test files are
   missing from the seed, so contributors without Ubicloud access learn within a