feat: Add .gitkeep files to empty toolbox directories and update QWEN.md files

- Add .gitkeep files to maintain empty toolbox-* directories in git
- Update top-level QWEN.md with project-wide guidelines
- Refine ToolboxStack/QWEN.md removing redundant content
- Add .gitkeep files to: toolbox-base, toolbox-docstack, toolbox-etl,
  toolbox-gis, toolbox-lifecycle-buildandtest,
  toolbox-lifecycle-packageandrelease, toolbox-weather

ToolboxStack/output/toolbox-base/.gitkeep

@@ -0,0 +1,2 @@
+# This file keeps the directory in git even when it's empty.
+# Actual implementation files will be added soon.

ToolboxStack/output/toolbox-docstack/.gitkeep

@@ -0,0 +1,2 @@
+# This file keeps the directory in git even when it's empty.
+# Actual implementation files will be added soon.

ToolboxStack/output/toolbox-etl/.gitkeep

@@ -0,0 +1,2 @@
+# This file keeps the directory in git even when it's empty.
+# Actual implementation files will be added soon.

ToolboxStack/output/toolbox-gis/.gitkeep

@@ -0,0 +1,2 @@
+# This file keeps the directory in git even when it's empty.
+# Actual implementation files will be added soon.

ToolboxStack/output/toolbox-lifecycle-buildandtest/.gitkeep

@@ -0,0 +1,2 @@
+# This file keeps the directory in git even when it's empty.
+# Actual implementation files will be added soon.

ToolboxStack/output/toolbox-lifecycle-packageandrelease/.gitkeep

@@ -0,0 +1,2 @@
+# This file keeps the directory in git even when it's empty.
+# Actual implementation files will be added soon.

ToolboxStack/output/toolbox-weather/.gitkeep

@@ -0,0 +1,2 @@
+# This file keeps the directory in git even when it's empty.
+# Actual implementation files will be added soon.