feat: update agents.md and add a feat (#128)

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

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.

* test(backend): add comprehensive dual-stack IP preference test suite

Added 43 tests covering:
- Core IP selection logic (selectTunnelDialHost)
- Node capability detection (nodeSupportsV4/V6)
- Address picker functions
- API contract tests for create/update/list
- Database compatibility (SQLite + PostgreSQL)

Fixed pre-existing broken test in federation_runtime_test.go

go-backend/internal/store/sqlite/sql/schema.sql

@@ -80,7 +80,8 @@ CREATE TABLE IF NOT EXISTS tunnel (
   updated_time INTEGER NOT NULL,
   status INTEGER NOT NULL,
   in_ip TEXT,
-  inx INTEGER NOT NULL DEFAULT 0
+  inx INTEGER NOT NULL DEFAULT 0,
+  ip_preference VARCHAR(10) NOT NULL DEFAULT ''
 );
 
 CREATE TABLE IF NOT EXISTS chain_tunnel (