Truncate tool result log output to 10 lines

[vm-mishchenko]

Summary

• Adds _truncate_tool_result helper in src/pipeline/runner.py that caps output at 10 lines and appends ... <N> more lines ... when truncated
• Calls the helper from the tool_result branch of _format_event only; all other branches are unchanged
• Reduces noise in live console and step log files caused by long Read file dumps and verbose Bash outputs

Test plan

• Direct unit tests for _truncate_tool_result: single line, 10-line boundary (no marker), 11-line truncation, 250-line truncation, empty string
• Integration tests for the [ok] branch of _format_event: 10-line passthrough, 11-line truncation, is_error: true still truncated, list-shaped content truncated
• Existing test_format_event_ok_full_multiline (3-line input) continues to pass unchanged