Catch British spellings locally instead of in the pipeline

Vale owns prose style and runs only in the merge request pipeline. That is fine for a rule that either fires or does not; the British-spelling rule is not one of those. It caught behaviour and not behaviours, then licence twice and not a third occurrence three lines on. A merge request goes green, the tree keeps the mistake, and the next contributor inherits a red default branch they did not cause — which happened twice this month (!24 (merged), !26 (merged)).

prose-check is an enumerated whole-word grep over the shipped markdown, wired into just ci. It does not reimplement vale; it turns "the pipeline found one instance" into "the tree has none".

The word list is enumerated rather than patterned deliberately: a \w+ise pattern would flag advertise, comprise, exercise, promise, supervise and surprise, all correct in US English. Add a word when vale teaches us one.

It found a hit on its first run, now fixed in the same change: a Bedrock model catalogue that AWS spells catalog.

Verified both directions, per the repo's own rule that a control has to be able to fail: injecting behaviour into MAINTENANCE.md fails the check with MAINTENANCE.md:133: behaviour and exit 1; restoring the file passes with exit 0. just ci green.