docs(agents): update knowledge base with encryption, API envelope, and build conventions (#127)

Add comprehensive documentation of project conventions including:
- Encryption patterns (AES with node secret PSK)
- API envelope structure (code, msg, data, ts)
- Build peculiarities (minify: false, rolldown-vite, UPX compression)
- Unique styles (flat monorepo, asymmetric Go layout, hybrid frontend mode)
- Module boundaries and anti-patterns
- Large file hotspots and code map references

Updated 7 AGENTS.md files across root and submodules.

go-backend/internal/http/handler/AGENTS.md

@@ -1,6 +1,6 @@
 # BACKEND HTTP HANDLER KNOWLEDGE BASE
 
-**Generated:** Fri Feb 13 2026
+**Generated:** Sun Feb 15 2026
 
 ## OVERVIEW
 HTTP request handlers for FLVX Admin API. Core business logic layer.
@@ -29,8 +29,9 @@ handler/
 
 ## CONVENTIONS
 - Inherits from parent: raw SQL, no ORM, JWT in Authorization header.
-- Large files expected (`mutations.go` >100k LOC).
+- Large files expected (`mutations.go` 3716 LOC - central mutation hub).
 - Uses `sqlite.Repository` for DB access via `repo.XXX()` methods.
+- Domain-driven file split: one file per functional area (federation, jobs, etc.).
 
 ## ANTI-PATTERNS
 - Do NOT add ORM here - uses raw SQL throughout.