fix(cron): run due jobs in parallel to prevent serial tick starvation (#13021)

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

hermes_cli/config.py

@@ -796,6 +796,11 @@ DEFAULT_CONFIG = {
         # Wrap delivered cron responses with a header (task name) and footer
         # ("The agent cannot see this message").  Set to false for clean output.
         "wrap_response": True,
+        # Maximum number of due jobs to run in parallel per tick.
+        # null/0 = unbounded (limited only by thread count).
+        # 1 = serial (pre-v0.9 behaviour).
+        # Also overridable via HERMES_CRON_MAX_PARALLEL env var.
+        "max_parallel_jobs": None,
     },
 
     # execute_code settings — controls the tool used for programmatic tool calls.