feat: add cwd (working directory) support for stdio MCP servers

[michaeljabbour]

Summary

Adds the ability to specify a working directory for stdio-based MCP server processes.

Changes

• Add cwd parameter to MCPClient constructor
• Pass cwd to StdioServerParameters when starting servers
• Support cwd in per-server config (e.g., in mcp.json or inline bundle config)
• Fall back to AMPLIFIER_WORK_DIRECTORY environment variable if not specified in config
• Update README with per-server configuration documentation

Use Case

This enables MCP servers like image generators (e.g., ImageGen) to save files to the user's project directory rather than the sidecar's working directory. Desktop applications can set AMPLIFIER_WORK_DIRECTORY based on the active conversation/project, and MCP servers will automatically use it.

Example Config

{
  "imagen": {
    "command": "npx",
    "args": ["-y", "@anthropics/imagen-mcp"],
    "cwd": "/path/to/project"
  }
}

Or use the environment variable fallback:

export AMPLIFIER_WORK_DIRECTORY=/path/to/project

Testing

• All 50 existing tests pass
• New feature is backward compatible (cwd defaults to None, preserving existing behavior)

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