[FEATURE] Add option to close previous tab when using /new command

Feature Request

Problem

When using the /new command to start a new conversation, it opens a new tab but leaves the previous conversation tab open. Over time, this leads to tab clutter as users accumulate multiple conversation tabs that they no longer need.

Current Workaround

Users must manually close unwanted tabs by clicking the X on each tab, or use /clear instead (which doesn't create a new tab but loses the conversation history in the current tab).

Proposed Solution

Add one of the following options:

Option 1: Command flag

/new --close-previous

This would open a new conversation tab and automatically close the previous one.

Option 2: Setting
Add a user preference/setting:

auto_close_previous_tab_on_new: true

When enabled, /new would automatically close the previous tab.

Option 3: New command
Create a separate command like /replace that combines the behavior of starting a new conversation and closing the current one.

Use Case

This is particularly useful for users whose typical workflow is:

1. Complete a task in one conversation
2. Start a fresh conversation for a new unrelated task
3. Don't need to reference the previous conversation

Currently, these users end up with many open tabs that need manual cleanup.

Alternative Considered

Using /clear works for starting fresh but doesn't provide the clean separation of a new tab, and the conversation history is immediately lost (can't undo if needed).