[cli] stale option names for "channel import" flow

### StatGPT Backend version

latest

### Short description

stale and uninformative "Clean up existing channels before import" option

### What steps will reproduce the bug?

run `channel import` command

### What is the expected behavior?

cli should inform what each proposed option does. specifically, "Clean up existing channels before import" does the following:

1. removes the imported channel (if present)
2. removes any resources (tables) linked to previous channel: search indices (elastic, postgres), channel-dataset mapping table, etc
3. creates a new channel

suggested name for this option: "Update existing channel and associated resources (search indices, channel-dataset mapping, etc)"

### What do you see instead?

3 options:

○ Clean up existing channels before import
○ Update existing datasets
○ Update existing data sources

### Additional information

_No response_

Start by running the `channel import` command and locate the code that displays the three existing option labels. Update the stale cleanup wording so it explains the channel and associated-resource behavior described in the issue, then rerun the command to verify that each proposed option is informative.

Written by the indexing model from the issue text.