fix(run): exit with non-zero code on session error

[zerone0x]

Problem

When opencode run encounters a session error (e.g., an invalid model specified via --model), the process exits with code 0 instead of 1.

$ opencode run --model "invalid-model-123" "test" 2>/dev/null; echo "EXIT: $?"
EXIT: 0  # Should be 1

Root Cause

loop() was fired with .catch() (not awaited), so after sdk.session.prompt() returned, execute() completed immediately without waiting for the event loop to finish. Even when the loop ran and set the error variable from a session.error event, process.exit(1) was never called because error was scoped inside the unawaited promise.

Fix

• Store the loop() promise as loopDone instead of calling .catch() immediately
• Await loopDone after the prompt/command is submitted (the loop terminates naturally when the session reaches idle state)
• Call process.exit(1) if the error variable was set during the session

This preserves the existing behavior of printing errors to stderr and calling process.exit(1) on unexpected loop exceptions.

Fixes #15558

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