All handled errors in --output-format json mode now write the structured
abort envelope to stdout (rc=1) and keep stderr empty. Previously the
top-level error handler and resume_session JSON branches used eprintln!
which sent the envelope to stderr, breaking machine consumers that read
stdout for command payloads.
Surfaces fixed:
- Top-level abort handler (main.rs): export --session <missing>,
session <subcommand>, prompt (no text), unknown subcommand fallthrough,
flag errors, and all other run() failures
- resume_session JSON branches: session load errors, unsupported commands,
parse errors, command execution errors
Test changes: updated 24 failing contract tests to assert JSON envelopes
on stdout. Added stderr-clean assertions where appropriate. 70 contract
tests pass (was 68; 2 additional from regression coverage).
ROADMAP: #819 (export session-not-found), #820 (interactive_only class),
#823 (missing prompt)
bc41e58b4f