refactor: replace server-mproc with subprocess.h
- Use vendor/sheredom/subprocess.h instead of custom process management
- Remove server-mproc.cpp (~600 lines) and server-mproc.h (~80 lines)
- Drop cwd support from mcp_server_config (subprocess.h limitation)
- Remove PYTHONUNBUFFERED auto-injection (user responsibility now)
Per @ngxson feedback to reuse existing subprocess.h.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
