docs: add description for MCP command

[cravenceiling]

This PR adds the missing opencode mcp entry to the opencode --help output.

The command is already implemented and usable, but it is currently not discoverable from the CLI help, which may confuse users.

Changes

• Added mcp command description to the CLI help output

Before

opencode mcp is not listed in --help

After

opencode mcp appears alongside other commands in the help output