docs: fix unsupported headers in JSON stdin OpenTelemetry example, clarify MCP_GATEWAY_PORT behavior

[Copilot]

Two documentation inaccuracies that cause user-facing failures or confusion:

1. The JSON stdin gateway.opentelemetry example included a headers field that the schema explicitly rejects (additionalProperties: false in opentelemetryConfig, no headers field in StdinOpenTelemetryConfig). Users following this example will get a validation error and a broken gateway startup.

2. The README implied awmg reads MCP_GATEWAY_PORT to set its listen address — it doesn't. Only run.sh/run_containerized.sh use it to construct the --listen flag.

Changes

• docs/CONFIGURATION.md

  • Annotate headers as TOML only in the OpenTelemetry field table (consistent with sample_rate)
  • Remove "headers" from the JSON stdin example block:

    // before
    {
      "gateway": {
        "opentelemetry": {
          "spanId": "00f067aa0ba902b7",
          "headers": "Authorization=******"  // ❌ not supported
        }
      }
    }
    
    // after — headers line removed; note added directing to OTEL_EXPORTER_OTLP_HEADERS

  • Add a blockquote note after the example pointing to OTEL_EXPORTER_OTLP_HEADERS as the correct mechanism

• README.md

  • Reword the port field description to accurately state that MCP_GATEWAY_PORT is consumed by wrapper scripts, not by awmg directly; --listen is the right flag

[Copilot]

Pull request overview

This PR corrects two documentation inaccuracies around OpenTelemetry JSON stdin configuration and how the gateway’s listen port is set, aiming to prevent validation errors and reduce user confusion when configuring/running awmg.

Changes:

• Clarifies that gateway.opentelemetry.headers is not supported in JSON stdin and removes it from the JSON stdin example in docs/CONFIGURATION.md, pointing users to OTEL_EXPORTER_OTLP_HEADERS instead.
• Rewords the README’s port field description to better explain that --listen controls the bind address and that wrapper scripts construct it from env vars.

Show a summary per file

File	Description
README.md	Updates the port field documentation to clarify how the listen address is set.
docs/CONFIGURATION.md	Fixes the OpenTelemetry JSON stdin example and labels headers as TOML-only, adding guidance for JSON stdin users.

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• Review effort level: Low