fix(cron): clean up HERMES_CRON_SESSION env var after each job run (#56771)

[hashbender]

Problem

When the cron scheduler runs in-process via InProcessCronScheduler (same Python process as the gateway), run_job() sets os.environ["HERMES_CRON_SESSION"] = "1" but never removes it in the finally block. Since os.environ is process-global and shared across all threads, this sentinel leaks into interactive gateway sessions.

Impact: After any cron job runs, execute_code and terminal commands in Telegram/gateway interactive sessions are blocked with the cron-deny approval error until the gateway process is restarted.

Root Cause

• cron/scheduler.py:2444 — sets os.environ["HERMES_CRON_SESSION"] = "1"
• cron/scheduler.py:3025+ — finally block cleans up TERMINAL_CWD, ContextVars, session DB, agent resources — but not HERMES_CRON_SESSION
• tools/approval.py:2655 — check_code_execution_approval() reads HERMES_CRON_SESSION from os.getenv() and blocks execution

The codebase already uses ContextVars for per-job session state to avoid exactly this class of cross-session pollution, but HERMES_CRON_SESSION was left as a raw os.environ write.

Fix

Add os.environ.pop("HERMES_CRON_SESSION", None) to the finally block in run_job(), alongside the existing TERMINAL_CWD cleanup. This ensures the env var is only present while a cron job is actively running.

Fixes NousResearch#56771

---

Mirror-of: NousResearch#57124
NousResearch#57124