Replaces the serial for-loop in tick() with ThreadPoolExecutor so all
jobs due in a single tick run concurrently. A slow job no longer blocks
others from executing, fixing silent job skipping (issue #9086).
Thread safety:
- Session/delivery env vars migrated from os.environ to ContextVars
(gateway/session_context.py) so parallel jobs can't clobber each
other's delivery targets. Each thread gets its own copied context.
- jobs.json read-modify-write cycles (advance_next_run, mark_job_run)
protected by threading.Lock to prevent concurrent save clobber.
- send_message_tool reads delivery vars via get_session_env() for
ContextVar-aware resolution with os.environ fallback.
Configuration:
- cron.max_parallel_jobs in config.yaml (null = unbounded, 1 = serial)
- HERMES_CRON_MAX_PARALLEL env var override
Based on PR #9169 by @VenomMoth1.
Fixes#9086
c86915024e