Summary

• When a CLI agent runtime exits with a non-zero exit code (or zero with no NDJSON output) and stderr has content, the runtime now emits a CLI_STDERR error event with the stderr message
• Adds stderr optional field to AgentRunResult so captured stderr is available for logging/diagnostics downstream
• Previously, stderr was captured into stderrChunks but never used — messages sent via channels would silently "disappear" with no error

Closes #374

Test plan

• Existing test updated: stderr on success exit with output still produces no error event
• New test: stderr included in done result when present
• New test: non-zero exit + stderr → emits CLI_STDERR error event
• New test: zero exit + no NDJSON output + stderr → emits CLI_STDERR error event
• New test: non-zero exit without stderr → no CLI_STDERR event
• Full test suite passes (868 tests across 93 files)
• Type-check, lint, format all clean

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