Timestamp every agent log record

[evgenyrp]

This helps with investigating latency based on agent traces:

############################################################
# /css/css-grid/alignment: analysis
# started 23:09:28.821 UTC
############################################################
23:09:29.769 [system] session started (model=claude-opus-5)

23:09:31.406 --- turn 1 ---
23:09:31.406 [agent] I'll start by querying Treeherder for the actual failure details.

23:09:34.912 --- turn 2 ---
23:09:34.912 [agent→tool] Bash
{ ... }
23:09:37.260 [tool←ok]
Metadata saved to: ...

...

============================================================
23:14:02.368 [done] turns=11 cost=$0.3592 elapsed=43s