fix(windows): suppress console window flash on subprocess spawns

Add creationflags=CREATE_NO_WINDOW to every Windows Popen call
across the terminal, process registry, code execution, and kanban
worker subsystems. Prevents visible CMD windows from flashing on
the user's desktop during agent operation.

Also adds the _IS_WINDOWS module constant to kanban_db.py where
it was missing, for consistency with the other patched files.

5 Popen sites across 4 files:
- tools/environments/local.py (terminal foreground spawn)
- tools/process_registry.py (background process spawn)
- tools/code_execution_tool.py (sandbox + interpreter probe)
- hermes_cli/kanban_db.py (kanban worker spawn)

tools/code_execution_tool.py

@@ -1238,6 +1238,7 @@ def execute_code(
             stderr=subprocess.PIPE,
             stdin=subprocess.DEVNULL,
             preexec_fn=None if _IS_WINDOWS else os.setsid,
+            creationflags=subprocess.CREATE_NO_WINDOW if _IS_WINDOWS else 0,
         )
 
         # --- Poll loop: watch for exit, timeout, and interrupt ---
@@ -1568,6 +1569,7 @@ def _is_usable_python(python_path: str) -> bool:
              "import sys; sys.exit(0 if sys.version_info >= (3, 8) else 1)"],
             timeout=5,
             capture_output=True,
+            creationflags=subprocess.CREATE_NO_WINDOW if _IS_WINDOWS else 0,
         )
         return result.returncode == 0
     except (OSError, subprocess.TimeoutExpired, subprocess.SubprocessError):