---

This section details on the original issue you should resolve

<issue_title>Document stdio command: examples, log behavior, and usage guidance</issue_title>
<issue_description>Parent: #1534

Problem

The stdio command is the most directly agent-relevant feature in Dev Proxy — it's how agents use Dev Proxy as an MCP proxy. Yet its help text is the barest of all commands:

Description:
  Proxy stdin/stdout/stderr of local executables

Usage:
  devproxy stdio <command>... [options]

Arguments:
  <command>  The command and arguments to execute

Options:
  -c, --config-file <config-file>    The path to the configuration file
  ...

An agent trying to use this has almost nothing to work from:

• No examples of actual usage
• No explanation of what "proxy stdin/stdout/stderr" means in practice
• No mention that logs go to a timestamped file (not console) to avoid interfering with stdio streams
• No mention of which plugins work in stdio mode (IStdioPlugin vs IPlugin)
• No mention that the @ character in package names (like @devproxy/mcp) is handled correctly

Rationale

The stdio command is the bridge between Dev Proxy and the MCP/agent ecosystem. If agents can't figure out how to use it from --help, they'll fail at the exact integration point that matters most.

Proposed solution

1. Expanded description:

   Proxy stdin/stdout/stderr of local executables.
   Dev Proxy intercepts and processes the stdio streams of the specified command,
   applying configured plugins (mocking, error simulation, etc.) to the traffic.
   Logs are written to a timestamped file (devproxy-stdio-YYYYMMDD-HHmmss.log)
   to avoid interfering with the proxied streams.
   

2. Examples in help text:

   Examples:
     devproxy stdio npx -y @devproxy/mcp                 Proxy MCP server
     devproxy stdio node server.js                        Proxy Node.js app
     devproxy stdio -c myconfig.json node app.js          With custom config
   

3. Document log file behavior — agents need to know where to find logs since they won't appear on console.

4. Document stdio error output — stderr is used for usage errors and exceptions, which is correct but should be stated.

Affected code

• DevProxy/Commands/StdioCommand.cs — expand description, add help text
  </issue_description>

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