add {file:path} config substitution for reading values from files

[RalphSu2]

Painpoint:
I have configure a dozen of mcp tools, and want to share with team member.
Currently mcp-cli support environment variable as place holder for token, but it's not common that environment variable been set (avoid env expose), configuration like opencode could reference a token file instead of hardcoding or env. If mcp-cli configuration could support file reference, it could make it easier to spread configuration