fix(logging): rotate exo's log file by size, not only when exo starts

[AlexCheema]

Problem

exo starts a new log file (~/.exo/exo_log/exo.log) each time it starts, and never during a run. A node that runs for days or weeks keeps one ever-growing file.

At default verbosity each request logs a few lines: the worker's plan, the task it starts (with its parameters), and KV cache activity. On a 2-node soak test, two Mac Studios (M3 Ultra) serving steady load of about 1 request a second, exo.log grew 8.5 MB an hour on each node, about 200 MB a day, with no limit.

Fix

Rotate the file when exo starts, as before, and also whenever it passes 50 MB. Rotated files are compressed with zstd, and the last 5 are kept, as before. So a node's logs are bounded at about 50 MB plus a few MB of archives.

Tests

test_log_rotation.py sets exo's logger up with a 20 KB limit:

• writing past the limit rotates the file more than once, and keeps at most 5 compressed archives;
• the live file stays near the limit, and the last line written is in it;
• a small log is only rotated at start.

A 24-hour soak with this change follows; I'll add its disk measurements here.

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