refactor(i18n): rename docs/plaene/ -> docs/plans/

Last German folder name in demo-epb. Pairs cleanly with docs/plans-md/
(markdown source) following the project convention. All references
in landing page generator, CI workflows, and cross-doc links updated.

README.md

@@ -63,7 +63,7 @@ python3 tools/render_plantuml.py
 ## Guided tour (~30 min)
 
 ### 1. Project planning (Word)
-`docs/plaene/`:
+`docs/plans/`:
 - **Project-Manual.docx** — Navigation guide, reading order, roles
 - **PID.docx** — What is built and why
 - **SWE-Plan.docx** — Language, standards, branching, reviews, coverage targets