process.run raises UnicodeEncodeError for an input string with a lone surrogate #760

run in dokimi/dokimi/process.py:190 encodes a string input with input.encode("utf-8") before it spawns the child (lines 746 and 777 do the same in the other entry points). A prompt built from a question or an artifact text with a \ud800 JSON escape reaches it. Repro: process.run(['cat'], input=json.loads('"n\\ud800"'), timeout=5, env=process.Env.inherit()) raises UnicodeEncodeError: 'utf-8' codec can't encode character '\ud800' in position N: surrogates not allowed at process.py:190. It is not a hash, so the fix is a decision: refuse the input with a message where the prompt is built, or encode with errors='replace' for the child's stdin (a lone surrogate has no UTF-8 form, so surrogatepass would send invalid UTF-8 to the child). Only line 190 was run; 746 and 777 were read.

Found while fixing #720. The fix's PR is #761.