fix(mcp): check args before use in mcp

[burmudar]

• go run ./cmd/src mcp would panic because we didn't check the arg length before using it - now we check it and print usage text
• make usage consistent and remove the dot

Test plan

tested locally

go run ./cmd/src mcp
NOTE: This command is still experimental
The 'mcp' command exposes MCP tools as subcommands for agents to use.

USAGE:
  src mcp list-tools              List available tools
  src mcp <tool-name> schema      View the input/output schema of a tool
  src mcp <tool-name> <flags>     Invoke a tool with the given flags
  src mcp <tool-name> -h          List the available flags of a tool