feat: add shell_exec tool and fix orchestrator model name resolution

- Add shell_exec to orchestrator tool suite (system.py + __init__.py)
  Runs arbitrary shell commands on the Cortex host with timeout (1–120s),
  combined stdout/stderr output, optional working_dir, and exit code reporting.
  Enables system diagnostics (df, ls, ps, journalctl, etc.) from Agent mode.

- Fix orchestrator_engine.run() to use model_name from resolved registry entry
  Previously used settings.orchestrator_model (.env hardcode) regardless of
  what model was assigned to the orchestrator role. Now accepts model_name param
  and falls back to settings value only when registry has no model_name.

- Update ARCH__FUTURE.md: date, running host, local orchestrator status,
  model registry V2 progress, added Cortex Mesh concept (section 9)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/orchestrator_engine.py

@@ -57,6 +57,7 @@ async def run(
     session_messages: list[dict] | None = None,
     respond_with_claude: bool = True,
     gemini_api_key: str | None = None,
+    model_name: str | None = None,
 ) -> OrchestratorResult:
     """
     Run the full orchestration loop for a task.
@@ -96,7 +97,7 @@ async def run(
 
         response = await asyncio.to_thread(
             client.models.generate_content,
-            model=settings.orchestrator_model,
+            model=model_name or settings.orchestrator_model,
             contents=contents,
             config=types.GenerateContentConfig(
                 tools=TOOL_DECLARATIONS,