integration: add docs/chat.md

docs/chat.md

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+# DeepSeek V4 Chat — Module Documentation
+
+Status: **integrated** (v1.0.0, 2026-08-09) · Registry: `modules/chat-module.yaml`
+
+## 1. Overview
+
+The chat module integrates the DeepSeek-V4-Sovereign model into the
+Sovereign stack as a conversational core: session management (bounded
+history), sovereign-context system prompt, OpenAI-compatible API client,
+and a deterministic local-echo fallback so the module runs — and is
+testable — with zero external dependencies.
+
+## 2. Architecture
+
+```
+chat/
+├── core.py            ChatMessage · ChatSession · DeepSeekV4Client
+└── api/
+    ├── routes.py      /api/v1/chat/* (5 endpoints)
+    └── app.py         standalone app (port 8003)
+site/chat.html         chat web UI (self-contained, responsive)
+```
+
+Wired into `src/sovereign/main.py` with a guarded include (same pattern
+as matrix/kyrexis/pec5d/coldcase).
+
+## 3. Behavior
+
+- **API mode** — `DEEPSEEK_API_KEY` set: OpenAI-compatible
+  `/chat/completions` call; errors surface as 502.
+- **Local mode** — no key: deterministic sovereign echo, no network.
+- **Sessions** — in-memory, bounded to `max_history` (50), system prompt
+  always retained.
+
+## 4. Configuration
+
+`config/chat.yaml` — model id/base/temperature/max_tokens, session bounds,
+system prompt. Keys via env only.
+
+## 5. Verification
+
+- `tests/test_chat.py` — 7 tests (defaults, sessions, bounding, local
+  echo, env config, model list, API surface).
+- Registry validator covers the `chat` module entry.
+
+## 6. Production notes
+
+- In production set `DEEPSEEK_API_KEY` (or a gateway URL) and serve behind
+  TLS; the UI (`site/chat.html`) talks to `/api/v1/chat` same-origin.
+- The local echo is a smoke path, not a production model answerer.