handleConn processed requests serially, so a slow tool (e.g.
wait_for_pattern with a 300s timeout) monopolized the single per-agent
MCP connection and every queued call timed out behind it. Handle each
request in its own goroutine, serialize responses through a per-conn
write mutex (full response written atomically, partial writes handled),
copy the request line before handing it off (bufio reuses its buffer),
and wait on a WaitGroup before closing the conn so in-flight handlers
finish cleanly. Greeting stays sequential; notifications still get no
response.
Resolves the [MCP TIMEOUT] TODO item.
