feat(cli): add /reasoning slash command to manage reasoning effort

Cherry-picked from PR #789 by Aum08Desai, rebased onto current main
with conflict resolution and improvements:

- Added /reasoning command: view current level or set to none|low|medium|high|xhigh
- Persists to config via save_config_value, forces agent re-init
- Resolved conflict with COMMANDS_BY_CATEGORY refactor (added to Configuration category)
- Restricted valid levels to none, low, medium, high, xhigh (removed 'minimal')
- Updated _parse_reasoning_config in cli.py and _load_reasoning_config in gateway/run.py
- Improved display messages (show all valid options, clearer defaults/disabled state)
- Added EXPECTED_COMMANDS entry for regression guard
- Expanded test suite: 16 tests covering all levels, rejection, display, case insensitivity,
  config save failure

Co-authored-by: Aum08Desai <145567217+Aum08Desai@users.noreply.github.com>

gateway/run.py

@@ -366,7 +366,7 @@ class GatewayRunner:
         """Load reasoning effort from config or env var.
         
         Checks HERMES_REASONING_EFFORT env var first, then agent.reasoning_effort
-        in config.yaml. Valid: "xhigh", "high", "medium", "low", "minimal", "none".
+        in config.yaml. Valid: "none", "low", "medium", "high", "xhigh".
         Returns None to use default (medium).
         """
         effort = os.getenv("HERMES_REASONING_EFFORT", "")
@@ -385,7 +385,7 @@ class GatewayRunner:
         effort = effort.lower().strip()
         if effort == "none":
             return {"enabled": False}
-        valid = ("xhigh", "high", "medium", "low", "minimal")
+        valid = ("low", "medium", "high", "xhigh")
         if effort in valid:
             return {"enabled": True, "effort": effort}
         logger.warning("Unknown reasoning_effort '%s', using default (medium)", effort)