docs(acp-nats-stdio): add README with configuration reference (#1)

Signed-off-by: Yordis Prieto <yordis.prieto@gmail.com>

Author: yordis

rsworkspace/crates/acp-nats-stdio/README.md

@@ -0,0 +1,80 @@
+# ACP NATS Stdio
+
+Translates [Agent Client Protocol](https://agentclientprotocol.com) (ACP) messages between stdio and [NATS](https://nats.io), letting IDEs and CLI tools talk to distributed agent backends without a direct network connection from the client.
+
+For managed NATS infrastructure in production, we recommend <a href="https://synadia.com"><img src="./assets/synadia-logo.png" alt="Synadia" width="20" style="vertical-align: middle;"> Synadia</a>.
+
+```mermaid
+graph LR
+    A[IDE] <-->|stdio| B[acp-nats-stdio]
+    B <-->|NATS| C[Backend]
+```
+
+## Features
+
+- Bidirectional ACP bridge with request forwarding
+- OpenTelemetry integration (logs, metrics, traces)
+- Graceful shutdown (SIGINT/SIGTERM)
+- Custom prefix support for multi-tenancy
+
+## Quick Start
+
+```bash
+docker run -p 4222:4222 nats:latest
+
+cargo build --release -p acp-nats-stdio
+
+./target/release/acp-nats-stdio
+```
+
+## Configuration
+
+### ACP
+
+| Variable | Description | Default |
+|----------|-------------|---------|
+| `ACP_PREFIX` | Subject prefix for multi-tenancy | `acp` |
+| `ACP_OPERATION_TIMEOUT_SECS` | Timeout for NATS request/reply operations | built-in default |
+| `ACP_PROMPT_TIMEOUT_SECS` | Timeout for prompt round-trips | built-in default |
+| `ACP_NATS_CONNECT_TIMEOUT_SECS` | NATS connection timeout | `10` |
+
+CLI flag `--acp-prefix` overrides `ACP_PREFIX`.
+
+### NATS
+
+| Variable | Description | Default |
+|----------|-------------|---------|
+| `NATS_URL` | Server URL(s), comma-separated for failover | `localhost:4222` |
+
+### NATS Authentication
+
+Resolved in priority order — the first match wins:
+
+| Priority | Variable(s) | Method |
+|----------|-------------|--------|
+| 1 | `NATS_CREDS` | Credentials file path |
+| 2 | `NATS_NKEY` | NKey seed |
+| 3 | `NATS_USER` + `NATS_PASSWORD` | Username/password |
+| 4 | `NATS_TOKEN` | Token |
+
+If none are set, the connection is unauthenticated.
+
+### Observability
+
+| Variable | Description |
+|----------|-------------|
+| `RUST_LOG` | Tracing filter directive (default: `info`) |
+| `ACP_LOG_DIR` | Directory for file-based logging |
+
+### OpenTelemetry
+
+Traces, metrics, and logs are exported over HTTP. The following [OTLP environment variables](https://opentelemetry.io/docs/specs/otel/protocol/exporter/) are supported:
+
+| Variable | Description |
+|----------|-------------|
+| `OTEL_EXPORTER_OTLP_ENDPOINT` | Collector base URL (e.g. `http://localhost:4318`) |
+| `OTEL_EXPORTER_OTLP_HEADERS` | Custom headers, comma-separated `key=value` pairs (e.g. auth tokens) |
+| `OTEL_EXPORTER_OTLP_TIMEOUT` | Export timeout in milliseconds (default: `10000`) |
+| `OTEL_RESOURCE_ATTRIBUTES` | Additional resource attributes, comma-separated `key=value` pairs |
+
+`OTEL_SERVICE_NAME` is hardcoded to `acp-nats-stdio` and cannot be overridden.