Add truncated option for long /run and /test outputs

[zlb22]

Summary

• truncate /run and /test outputs when they exceed 400 lines or 32 KB of text before adding to chat
• keep head/tail plus up to 3 regex-matched blocks (±20 lines) with gap markers and a header "[truncated: total X lines, kept Y]"
• keep terminal output full; prompt [T]/[F]/[N] (default truncated); /test failures use truncated by default

Testing

• pytest tests/basic/test_commands.py -k truncates_long_output
• pre-commit run --all-files

Fixes #4730