fix: process.stdout.write instead of console.log for export cmd

[veracioux]

This makes sure that all of the output is written before the command exits.

Verified by:

opencode export <sessionID>: produces expected output.
opencode export <sessionID> | jq: produces expected output.

For very large data, one could try to mock a session with a lot of data. I opted
for the simpler approach of just replacing the changed code with the following:

const data = 'a'.repeat(1024**3) // 1GiB of data should be more than enough safety margin
process.stdout.write(JSON.stringify(exportData, null, 2))

and running opencode export <sessionID> | wc -c, which printed 1073741824,
which indeed corresponds to 1GiB.