feat: add 'never' option for mcp_timeout to disable timeout

[Skyline-23]

Problem

Currently, mcp_timeout in experimental config only accepts positive integers (milliseconds). There's no way to completely disable timeout for MCP tool calls.

Some MCP tools may take a long time to execute (e.g., long-running browser automation, complex code analysis), and users need an option to disable the timeout entirely.

Proposed Solution

Allow mcp_timeout to accept "never" as a value to disable timeout:

{
  "experimental": {
    "mcp_timeout": "never"
  }
}

Alternatives Considered

• Using a very large number (e.g., 999999999) - not intuitive
• A separate boolean flag - adds complexity