fix: keep API error output on stderr

[fallintoplace]

Summary

• write formatted API error bodies to stderr alongside the HTTP status line
• fall back to JSON for --format-error explore, since the interactive explorer writes to stdout
• add an end-to-end regression test that keeps stdout empty on API failures

Why

API failures currently split their output across streams: the status line goes to stderr, while the structured response body defaults to stdout. This makes failed commands unsafe to pipe because error JSON can be mistaken for successful output.

Test plan

• ./scripts/test
• regression test covers exit status, empty stdout, and the status plus JSON body on stderr