ext/exec: keep failure output reachable by read_tool_result (MaxOutput vs offload) #59

Leftover from #41 (closed), which landed read_tool_result grep with context in #49.

exec.Config.MaxOutput defaults to 32 KiB, and capBuffer drops the middle inside Source.Call, upstream of OffloadingSource. On a long go test transcript the failing assertion is cut before any store sees it, so grep-with-context has nothing to find. The mechanism exists and is unreachable from exactly the path it was built for.

Decide, don't just raise a number

• MaxOutput is a resource guard on what a runaway process can put in memory. It stays finite.
• The offload threshold decides what reaches context inline.

Pick a default where a realistic failing go test ./... transcript reaches the store whole, while context still gets only a reduced view. Then test it: a failing test whose assertion sits in the middle of a large transcript, found by read_tool_result grep.

The decision on a per-turn state summary from #41 stays open and isn't in scope here.