Summary

• Switches the issue datetime.utcnow() deprecated + naive/aware timestamp islands across session/goal/channel layers #395 timestamp paths from naive timestamps to timezone-aware UTC values.
• Removes deprecated datetime.utcnow() usage.
• Adds a focused regression test for session model timestamp serialization.

Why

Fixes #395.

This avoids deprecated UTC timestamp generation and makes stored/generated timestamps explicit about their timezone.

Changes

• Added a small UTC timestamp helper in the session models.
• Updated session, goal, channel, API, swarm, and connector timestamp generation to use timezone-aware UTC.
• Added a grep gate for datetime.utcnow() and bare datetime.now() in the scoped timestamp paths.
• Added agent/tests/test_session_timestamp_timezone.py.

Test Plan

• python3 -m pytest -q agent/tests/test_session_timestamp_timezone.py
• bash tools/ci_grep_gates.sh
• grep -RIn "datetime.utcnow(" agent tools || true
• git diff --check