Reorganize brain: projects/ top level, rename filenames, update homepage

- Moved everything from ideas/passepartout/ to projects/passepartout/
- Moved legal structures to projects/flags/
- Created missing _index.org files for all subdirectories
- Stripped redundant passepartout- prefix from filenames
- Rewrote root _index.org as generalized brain index (projects + concepts)
- Updated Hugo nav to Projects/Concepts
- Updated build script section descriptions
- Deleted stale ideas/passepartout-economics.md orphan

projects/flags/_index.org

@@ -0,0 +1,8 @@
+:PROPERTIES:
+:CREATED:  [2026-05-24 Sun]
+:ID:       1e5f6a7b-8c9d-0e1f-2a3b-4c5d6e7f8a9b
+:END:
+#+title: Flags — Legal Structures
+#+filetags: :index:
+
+Legal structure analysis for the Passepartout project — entity types, jurisdictional considerations, asset protection, practical setup guides.