ensureStarted's stderr-copy goroutine could still be mid-Read when close()
called cmd.Wait(), which os/exec's StderrPipe docs call out as incorrect
and can truncate/garble trailing stderr diagnostics or surface a spurious
"file already closed" error. close() now waits on a stderrDone channel,
closed by the copy goroutine once it hits EOF (unblocked by killing the
process), before calling Wait.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
subprocessClient spawns the embedded pyscript/wrapper.py over os/exec and
speaks newline-delimited JSON instead of MCP. Auth/data methods land in
follow-up commits; this is the transport + lifecycle plumbing only.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
