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/_index.org

@@ -0,0 +1,8 @@
+:PROPERTIES:
+:CREATED:  [2026-05-24 Sun]
+:ID:       a0b1c2d3-e4f5-6a7b-8c9d-0e1f2a3b4c5d
+:END:
+#+title: Projects
+#+filetags: :index:
+
+All projects documented in this brain. Each project has its own architecture, strategy, and reference material.