ci(free-tests): restore the PR-number concurrency + failure-log artifact; truth-fix stale comments

The workspace-revert incident that hit CHANGELOG/TODOS mid-ship also
caught free-tests.yml between edits: commit 8d6c2ff8's message claims
PR-number concurrency + artifact upload + main-push runs, but only the
push trigger survived to the commit (caught by the /document-release
doc-vs-code audit). Both re-applied. Also: eval-model.ts header said
capture defaults to Opus (it's Sonnet per D1a), paid-shards' header
pinned a stale 44/63 shard census, and two CHANGELOG phrases
over-claimed ('six' -> 'up to six' shard processes; retry-1 scoped to
retry-bearing paid paths).

scripts/test-paid-shards.ts

@@ -23,9 +23,9 @@
  *   3. No per-shard env / eval dir. Each shard needs its own GSTACK_EVAL_DIR
  *      so eval baselines are per-test-file instead of last-flush-wins.
  *
- * Worst-case wall clock = ceil(shards / jobs) × shard timeout. At the time of
- * writing: gate is 44 one-file shards → ceil(44/4) × 30min = 5.5h; periodic is
- * 63 → 8h. Do NOT hand-derive the eval:bg:* detach timeouts from a snapshot of
+ * Worst-case wall clock = ceil(shards / jobs) × shard timeout. Shard counts
+ * drift as test files land, so treat any number written here as stale.
+ * Do NOT hand-derive the eval:bg:* detach timeouts from a snapshot of
  * these counts — test/eval-detach-timeout-floor.test.ts recomputes the bound
  * from the live shard census every run and fails CI if package.json's numbers
  * dip below it (undersized detach timeouts recreate never-started truncation).