[codex] Allow gateway to preserve suspended sessions

[juanfradb]

Summary

• add session_reset.reset_suspended to control whether stopped/interrupted gateway sessions are hard-reset
• when disabled, clear the suspended flag and mark the session resume_pending so the next message continues with the existing transcript
• document the setting and cover the default and opt-in behavior with focused tests

Why

The existing hard-suspend path was introduced to avoid unrecoverable stuck-session loops (#7536). Later restart-resume work added resume_pending for interrupted restarts (#11852), but explicit suspended sessions still always force a fresh session. That is safe, but it can be too destructive for deployments that prefer continuity and explicit /reset.

This PR keeps upstream default behavior unchanged (reset_suspended: true) and adds an opt-in for deployments that want stopped/interrupted sessions to continue automatically.

Validation

• /home/juan/.hermes/hermes-agent/venv/bin/python -m pytest tests/gateway/test_config.py::TestSessionResetPolicy tests/gateway/test_restart_resume_pending.py::TestGetOrCreateResumePending tests/gateway/test_session_reset_notify.py::TestResetPolicyNotify -q

Related: #7536, #4493, #9850, #9934, #11852

[teknium1]

Thanks for the session-continuity work. Current main already provides the requested behavior, so this is redundant as an automated hermes-sweeper review.

• Commit 952a885fbfa2b7c4f12791bfeead6d25bb361036 changed /stop to preserve the session; the current gateway handler at gateway/run.py:9237 interrupts and releases the running session without suspending it.
• Forced restart/shutdown interruptions are marked resume_pending at gateway/run.py:8170, and gateway/session.py:1772 returns the existing session entry and transcript on recovery.
• The remaining suspended state is the terminal three-restart stuck-loop safeguard (gateway/run.py:6088). The proposed reset_suspended: false branch would relax that safety boundary rather than restore normal stop/restart continuity.

No release tag was identified for the relevant changes.