Add MkDocs Material documentation site with 40 pages and auto-generated API reference

Sets up a complete documentation website with 7 navigable sections (Home, Getting
Started, User Guide, Architecture, API Reference, Deployment, Development), light/dark
mode, search, code copy, and Mermaid diagram support. API reference pages use
mkdocstrings to auto-generate docs from source docstrings. GitHub Actions workflow
deploys to GitHub Pages on push to main.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/api/engine.md

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+# Engine Module
+
+The engine module implements the inference runtime pillar. All backends
+implement the `InferenceEngine` ABC with `generate()`, `stream()`,
+`list_models()`, and `health()` methods. The discovery subsystem probes
+running engines and selects the best available backend based on
+configuration and health checks.
+
+## Abstract Base Class
+
+### InferenceEngine
+
+::: openjarvis.engine._stubs.InferenceEngine
+    options:
+      show_source: true
+      members_order: source
+
+### EngineConnectionError
+
+::: openjarvis.engine._base.EngineConnectionError
+    options:
+      show_source: true
+
+### messages_to_dicts
+
+::: openjarvis.engine._base.messages_to_dicts
+    options:
+      show_source: true
+
+---
+
+## Engine Implementations
+
+### OllamaEngine
+
+::: openjarvis.engine.ollama.OllamaEngine
+    options:
+      show_source: true
+      members_order: source
+
+### VLLMEngine
+
+::: openjarvis.engine.vllm.VLLMEngine
+    options:
+      show_source: true
+      members_order: source
+
+### LlamaCppEngine
+
+::: openjarvis.engine.llamacpp.LlamaCppEngine
+    options:
+      show_source: true
+      members_order: source
+
+### SGLangEngine
+
+::: openjarvis.engine.sglang.SGLangEngine
+    options:
+      show_source: true
+      members_order: source
+
+### CloudEngine
+
+::: openjarvis.engine.cloud.CloudEngine
+    options:
+      show_source: true
+      members_order: source
+
+### estimate_cost
+
+::: openjarvis.engine.cloud.estimate_cost
+    options:
+      show_source: true
+
+---
+
+## Engine Discovery
+
+Functions for probing running engines, aggregating available models,
+and selecting the best engine for a given configuration.
+
+### get_engine
+
+::: openjarvis.engine._discovery.get_engine
+    options:
+      show_source: true
+
+### discover_engines
+
+::: openjarvis.engine._discovery.discover_engines
+    options:
+      show_source: true
+
+### discover_models
+
+::: openjarvis.engine._discovery.discover_models
+    options:
+      show_source: true