fix(tools): remove unnecessary crontab requirement from cronjob tool (#1638)

* fix(tools): remove unnecessary crontab requirement from cronjob tool

The hermes cron system is internal — it uses a JSON-based scheduler
ticked by the gateway (cron/scheduler.py), not system crontab.

The check for shutil.which('crontab') was preventing the cronjob tool
from being available in environments without crontab installed (e.g.
minimal Ubuntu containers).

Changes:
- Remove shutil.which('crontab') check from check_cronjob_requirements()
- Remove unused shutil import
- Update docstring to clarify internal scheduler is used
- Update tests to reflect new behavior and add coverage for all
  session modes (interactive, gateway, exec_ask)

Fixes #1589

* test: add HERMES_EXEC_ASK coverage for cronjob requirements

Adds missing test for the exec_ask session mode, complementing
the cherry-picked fix from PR #1633.

---------

Co-authored-by: Bartok9 <bartokmagic@proton.me>

tools/cronjob_tools.py

@@ -8,7 +8,6 @@ Compatibility wrappers remain for direct Python callers and legacy tests.
 import json
 import os
 import re
-import shutil
 import sys
 from pathlib import Path
 from typing import Any, Dict, List, Optional
@@ -414,13 +413,10 @@ def check_cronjob_requirements() -> bool:
     """
     Check if cronjob tools can be used.
 
-    Requires 'crontab' executable to be present in the system PATH.
     Available in interactive CLI mode and gateway/messaging platforms.
+    The cron system is internal (JSON file-based scheduler ticked by the gateway),
+    so no external crontab executable is required.
     """
-    # Ensure the system can actually install and manage cron entries.
-    if not shutil.which("crontab"):
-        return False
-
     return bool(
         os.getenv("HERMES_INTERACTIVE")
         or os.getenv("HERMES_GATEWAY_SESSION")