docs: state both worker defaults with their lower clamp (#2336)

Copilot review round 2: both expressions omitted Vitest's `max(…, 1)`, so
as written they described zero workers on a one-core runner. Now
`max(availableParallelism() - 1, 1)` for the forks pool and
`max(min(12, availableParallelism() - 1), 1)` for the browser pool, and the
browser pool's single-worker condition names all three of its terms
(headless, file parallelism, provider support).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PsmUAZ6aLUAFqenjJKSSRp
Signed-off-by: cliffhall <cliff@futurescale.com>

AGENTS.md

@@ -458,9 +458,9 @@ free to run a full `local:gate`). Raising a budget nobody chose hides no race.
   option, a `describe` option, a project setting and a `--retry` flag all land
   on the same check.
 - **`maxWorkers` stays unset — on measurement, not by omission (#2336).** The
-  five forks-pool projects inherit `availableParallelism() - 1`; web's
+  five forks-pool projects inherit `max(availableParallelism() - 1, 1)`; web's
   `storybook` project runs the browser pool, whose own default is
-  `min(12, availableParallelism() - 1)` and which was not measured (a cap set
+  `max(min(12, availableParallelism() - 1), 1)` and which was not measured (a cap set
   there would apply, so it is not exempt — it just needs its own numbers).
   Capping the forks pool to 4 was
   measured at the leased baseline as +14% wall on the web unit suite and +39%