docs: Unify guidance — AGENTS.md as source of truth

Establish clear governance hierarchy:
1. AGENTS.md — AI coding constitution, authoritative
2. CLAUDE.md — Project context, supplements AGENTS.md
3. Code — Implementation, must comply with AGENTS.md

Changes:
- AGENTS.md: Add explicit declaration that this is the authoritative source
- CLAUDE.md: Add governance statement, redirect database config to AGENTS.md
- Quick Start: Remove stale/incorrect database credentials, point to appsettings.Development.json

Prevent guidance fragmentation:
- Do not add conflicting rules to CLAUDE.md
- All harnesses and procedures go to AGENTS.md
- Supplementary info only in CLAUDE.md (status, architecture, overview)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

AGENTS.md

@@ -1,4 +1,13 @@
-# K-ArtSell Aegis AI Coding Constitution v12.0
+# K-ArtSell Aegis AI Coding Constitution v16.0
+
+**AGENTS.md IS THE AUTHORITATIVE SOURCE FOR ALL ENGINEERING DECISIONS.**
+
+This document supersedes conflicting guidance in CLAUDE.md, MEMORY, or prior conversations. When in doubt, follow AGENTS.md.
+
+**Hierarchy:**
+1. **AGENTS.md** — This file (AI coding constitution, rules, harnesses)
+2. **CLAUDE.md** — Supplements AGENTS.md (project context, architecture, status)
+3. **Code** — Implementation (must comply with AGENTS.md)
 
 ## Default execution procedure
 

CLAUDE.md

@@ -1,6 +1,13 @@
 # CLAUDE.md
 
-This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+This file provides project context and architecture for Claude Code (claude.ai/code).
+
+**IMPORTANT: All engineering decisions are governed by AGENTS.md v16.0. This file supplements AGENTS.md but DOES NOT override it.**
+
+**Single Source of Truth (Hierarchy):**
+1. **AGENTS.md** — AI coding constitution, principles, rules (authoritative)
+2. **CLAUDE.md** — Project context, status, architecture (supplementary)
+3. **Project code** — Implementation (must comply with AGENTS.md)
 
 ## ⚖️ Governance: AGENTS.md v16.0 Strategic Principles
 
@@ -169,39 +176,21 @@ Invoke-WebRequest -Uri "http://127.0.0.1:5002/api/shadow-runs" `
 
 ## Quick Start
 
+**→ See AGENTS.md v16.0 "Development Environment Configuration" for authoritative setup.**
+
+This section follows AGENTS.md. Do not deviate.
+
 ### Prerequisites
 - .NET 10 SDK
 - Node.js 22 / pnpm 10
 - SSH access to remote PostgreSQL server (178.104.200.7)
 
-### Remote PostgreSQL Setup via SSH Port Forwarding
+### Local Development Commands
-
-The project database is hosted on `178.104.200.7`. Connect via SSH port forwarding:
 
 ```bash
+# SSH Tunnel (required first, in separate terminal)
 ssh -L 5432:127.0.0.1:5432 kjh2064@178.104.200.7
-```
 
-This command:
-- Forwards local port 5432 to remote PostgreSQL (127.0.0.1:5432)
-- Keeps the tunnel open while you develop
-- Run in a separate terminal/window and keep it running during development
-
-**Windows (PowerShell):**
-```powershell
-ssh -L 5432:127.0.0.1:5432 kjh2064@178.104.200.7
-```
-
-**macOS/Linux:**
-```bash
-ssh -L 5432:127.0.0.1:5432 kjh2064@178.104.200.7
-```
-
-Once the tunnel is open, your local `localhost:5432` connects to the remote database.
-
-### Local Development Environment
-
-```bash
 # Backend: restore, build, migrate, test
 dotnet restore KArtSell.sln
 dotnet build KArtSell.sln -c Release
@@ -229,25 +218,12 @@ pnpm dev  # Backend in another terminal
 ```
 
 ### Database Connection
-```
-Host: localhost
-Port: 5432
-Database: kartsell
-User: kartsell
-Password: kartsell
-```
 
-Environment variable: `KARTSELL_POSTGRES=Host=localhost;Port=5432;Database=kartsell;Username=kartsell;Password=kartsell`
+**Read from `src/KArtSell.Host/appsettings.Development.json` (source of truth).**
 
-**On Windows (PowerShell):**
+Do NOT use environment variables or make up credentials. Backend reads from config file.
-```powershell
-$env:KARTSELL_POSTGRES="Host=localhost;Port=5432;Database=kartsell;Username=kartsell;Password=kartsell"
-```
 
-**On macOS/Linux (bash):**
+Database is accessed through SSH tunnel only.
-```bash
-export KARTSELL_POSTGRES="Host=localhost;Port=5432;Database=kartsell;Username=kartsell;Password=kartsell"
-```
 
 ## Architecture