docs: add SYSTEM.md — agent constitution

Every agent must read this before writing code.
Defines rules, architecture, and quality standards.

SYSTEM.md

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+# SYSTEM DIRECTIVES — Read Before Any Action
+
+You are an agent working on The Nexus, Timmy's sovereign 3D home.
+
+## Rules
+
+1. READ this file and CLAUDE.md before writing any code.
+2. ONE PR at a time. Merge your open PRs before starting new work.
+3. Never submit empty or placeholder PRs. Every PR must change real code.
+4. Every PR must pass: `node --check` on all JS files, valid JSON, valid HTML.
+5. Branch naming: `{your-username}/issue-{N}`. No exceptions.
+6. Commit format: `feat:`, `fix:`, `refactor:`, `test:` with `Refs #N`.
+7. If your rebase fails, start fresh from main. Don't push broken merges.
+8. The acceptance criteria in the issue ARE the definition of done. If there are none, write them before coding.
+
+## Architecture
+
+- app.js: Main Three.js scene. Large file. Make surgical edits.
+- style.css: Cyberpunk glassmorphism theme.
+- index.html: Entry point. Minimal changes only.
+- ws-client.js: WebSocket client for Hermes gateway.
+- portals.json, lora-status.json, sovereignty-status.json: Data feeds.
+
+## Sovereignty
+
+This project runs on sovereign infrastructure. No cloud dependencies.
+Local-first. Bitcoin-native. The soul is in SOUL.md — read it.
+
+## Quality
+
+A merged PR with bugs is worse than no PR at all.
+Test your code. Verify your output. If unsure, say so.