feat: add ISO 8601 timestamps to deploy log entries
Prefix all deployment log lines with UTC timestamps in ISO 8601 format.
Rename [sower] prefix to [agent] for consistency. Drop log level from
activator deploy log output (stderr logging retains levels).
Format: "2026-03-18T12:34:56Z [agent] message"
Format: "2026-03-18T12:34:56Z [activator] message"
sow-60
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>