add interactive gateway demo

Introduces demos/gateway/ with a step-by-step shell demo for the
model-cli gateway command. Each step pauses for Enter, types commands
character-by-character as 'docker model gateway', and covers health,
auth, chat completions, streaming, embeddings, load balancing,
fallbacks, and OpenAI SDK compatibility.

Author: ericcurtin

demos/gateway/README.md

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+# model-cli gateway demo
+
+Demonstrates the `model-cli gateway` command — a lightweight,
+OpenAI-compatible LLM proxy that sits in front of Docker Model Runner
+(and other providers) and adds routing, load balancing, retries,
+fallbacks, and auth.
+
+## Prerequisites
+
+1. Docker Desktop with Model Runner enabled
+2. The `model-cli` binary built:
+   ```bash
+   cd model-cli && cargo build --release
+   ```
+3. Models pulled:
+   ```bash
+   docker model pull ai/smollm2
+   docker model pull ai/gemma3
+   docker model pull ai/qwen3:0.6B-Q4_0
+   docker model pull ai/nomic-embed-text-v1.5
+   ```
+4. Python `openai` package (for step 11):
+   ```bash
+   pip install openai
+   ```
+
+## Run the demo
+
+```bash
+./demos/gateway/demo.sh
+```
+
+The script starts the gateway on `http://localhost:4000`, runs through
+every feature, then shuts the gateway down on exit.
+
+## Files
+
+| File | Purpose |
+|------|---------|
+| `config-basic.yaml`    | Single-provider config with two models and bearer-token auth |
+| `config-advanced.yaml` | Multi-deployment config showing load balancing and fallbacks |
+| `demo.sh`              | Full end-to-end demo script |
+
+## What is demonstrated
+
+| # | Feature | Config |
+|---|---------|--------|
+| 1 | Start gateway | basic |
+| 2 | `/health` endpoint | basic |
+| 3 | `/v1/models` — OpenAI-compatible model list | basic |
+| 4 | Auth rejection with wrong key (HTTP 401) | basic |
+| 5 | Non-streaming chat completion | basic |
+| 6 | Streaming chat completion (SSE) | basic |
+| 7 | Embeddings via chat model | basic |
+| 8 | Switch to advanced config | advanced |
+| 9 | Round-robin load balancing across two deployments | advanced |
+| 10 | Dedicated embedding model (`nomic-embed-text`) | advanced |
+| 11 | OpenAI Python SDK — zero code changes required | advanced |
+
+## Config anatomy
+
+```yaml
+model_list:
+  # Alias the client uses       Provider / actual model on DMR
+  - model_name: fast-model
+    params:
+      model: docker_model_runner/ai/smollm2
+
+  # Second entry with same alias → round-robin load balancing
+  - model_name: fast-model
+    params:
+      model: docker_model_runner/ai/qwen3:0.6B-Q4_0
+
+  - model_name: big-model
+    params:
+      model: docker_model_runner/ai/gemma3
+
+general_settings:
+  master_key: demo-secret   # Bearer token required on all requests
+  num_retries: 2            # retry up to 2 times before fallback
+  fallbacks:
+    - fast-model: [big-model]   # automatic fallback chain
+```
+
+## Manual curl examples
+
+```bash
+GW="http://localhost:4000"
+KEY="demo-secret"
+
+# Health
+curl "${GW}/health"
+
+# List models
+curl -H "Authorization: Bearer ${KEY}" "${GW}/v1/models"
+
+# Chat completion
+curl -X POST "${GW}/v1/chat/completions" \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer ${KEY}" \
+  -d '{"model":"smollm2","messages":[{"role":"user","content":"Hello!"}]}'
+
+# Streaming
+curl -N -X POST "${GW}/v1/chat/completions" \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer ${KEY}" \
+  -d '{"model":"smollm2","messages":[{"role":"user","content":"Count to 5"}],"stream":true}'
+
+# Embeddings
+curl -X POST "${GW}/v1/embeddings" \
+  -H "Content-Type: application/json" \
+  -H "Authorization: Bearer ${KEY}" \
+  -d '{"model":"embeddings","input":["hello world"]}'
+```

demos/gateway/config-advanced.yaml

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+# Advanced gateway config: load balancing, retries, and fallbacks
+#
+# Demonstrates three key gateway features:
+#
+# 1. LOAD BALANCING — two deployments under the same model alias.
+#    The gateway round-robins across them automatically.
+#
+# 2. RETRIES — if a provider call fails, it will be retried up to
+#    num_retries times before giving up (or falling back).
+#
+# 3. FALLBACKS — if "fast-model" exhausts all retries, the gateway
+#    automatically promotes the request to "big-model".
+
+model_list:
+  # Two local DMR models registered under the same alias.
+  # Requests for "fast-model" are round-robined across both.
+  - model_name: fast-model
+    params:
+      model: docker_model_runner/ai/smollm2
+
+  - model_name: fast-model
+    params:
+      model: docker_model_runner/ai/qwen3:0.6B-Q4_0
+
+  # Larger fallback model
+  - model_name: big-model
+    params:
+      model: docker_model_runner/ai/gemma3
+
+  # Embedding model
+  - model_name: embeddings
+    params:
+      model: docker_model_runner/ai/nomic-embed-text-v1.5
+
+general_settings:
+  master_key: demo-secret
+  num_retries: 2                        # retry failing calls twice before fallback
+  fallbacks:
+    - fast-model: [big-model]           # fast-model falls back to big-model

demos/gateway/config-basic.yaml

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+# Basic gateway config: single Docker Model Runner provider
+#
+# The gateway exposes a unified OpenAI-compatible API on :4000 and
+# forwards requests to Docker Model Runner running on the local engine.
+
+model_list:
+  - model_name: smollm2        # alias clients use in their requests
+    params:
+      model: docker_model_runner/ai/smollm2   # provider/model
+
+  - model_name: gemma3
+    params:
+      model: docker_model_runner/ai/gemma3
+
+general_settings:
+  master_key: demo-secret      # clients must send: Authorization: Bearer demo-secret