coderai now always boots as the front proxy + supervised engine subprocess(es);
the one-process "UI/API and model work in the same process" mode is gone. This
unblocks removing the engine's now-dead UI/stats/data handlers (the engine only
ever runs as --engine-only behind the front).

Removed: the --single-process CLI flag (cli.py), config.server.single_process
(config.py + save), the proc-title branch and the run-mode branch (main.py), and
the arg from the engine spawn filter (engine_supervisor.py). Existing config.json
files with a stale single_process key are tolerated (_dc ignores unknown keys).

Verified: argparse rejects --single-process; --engine-only still parses.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011DDv7BchtZQWsnPG6Jm49m