perf: serialize tool results compactly

The foundations skill specifies compact serialization in ok()/fail().
Measured across the fleet, `null, 2` costs 19-41% of every response - worst
on wide lists of short-keyed objects, which are exactly the replies already
big enough to hurt. Measured end to end on mcp-npm against a live API: 25%
across five default calls.

No model needs the indentation. okText() stays raw so a markdown body is not
escaped into one unreadable line, and files written to disk for humans stay
pretty-printed.