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/passepartout/social-protocol/requirements-00-readme.org

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+#+title: Social Protocol: Decentralized Social Network
+#+AUTHOR: Amr
+#+CREATED: [2026-03-17 Tue]
+#+BEGIN_COMMENT
+A decentralized social network protocol for the ATmosphere (AT Protocol) ecosystem.
+#+END_COMMENT
+
+:PROPERTIES:
+:CREATED:  [2026-05-24 Sun]
+:ID:       10289e64-a4ff-4c34-828f-f3a9c769b73d
+:END:
+* [[id:1d074690-a279-59cb-b91d-e9a22ae104ad][Social Protocol]]: Decentralized Social Network
+
+This project contains the specification and analysis for a decentralized social network built on open protocols.
+
+* Project Tasks
+
+See the actionable tasks for this project in GTD.org (Social Protocol project)
+
+* Key Documents
+
+- [[id:b25bf753-9799-41ab-82f5-1a1416db756b][01. Overview]]
+- [[id:6fe67db6-25bd-4d11-bd1d-b44ec809e858][02. Identity]]
+- [[id:3b43a9b8-31d1-4479-a35f-22273b74f0c7][03. Infrastructure]]
+- [[id:f6cfc54b-919b-4311-bcbf-65e976755d40][04. The Primitive]]
+- [[id:0f949f6c-4cf1-49eb-b9a4-ebcac27ee548][05. Social Spaces]]
+- 
+- 
+- 
+- 
+- 
+- 
+- 
+- 
+
+* Status
+
+- [X] Concept and atomic notes complete
+- [X] Comprehensive specification built
+- [ ] Governance decision (DEC-001) pending
+- [ ] Implementation planning not started