Archive session on /clear instead of backgrounding

[Copilot]

The /clear command now archives the current session before creating a new one, distinguishing explicit clearing from passive session switching (plus button).

Changes

Modified the /clear slash command handler in chat.contribution.ts:

• Injected IAgentSessionsService to access session state
• Updated callback to receive sessionResource parameter and archive session via setArchived(true) before executing ACTION_ID_NEW_CHAT
• Added null check and archive state guard

}, async (_prompt, _progress, _history, _location, sessionResource) => {
    const currentSession = agentSessionsService.getSession(sessionResource);
    if (currentSession && !currentSession.isArchived()) {
        currentSession.setArchived(true);
    }
    commandService.executeCommand(ACTION_ID_NEW_CHAT);
}));

Original prompt

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This section details on the original issue you should resolve

<issue_title>/clear should archive current session and start a new one</issue_title>
<issue_description>

Does this issue occur when all extensions are disabled?: Yes/No

Currently the plus button creates a new session and keeps the current one running in the background. But /clear is an explicit action to clear content, so it should archive the current session rather than just pushing it to the background IMHO.
</issue_description>

Comments on the Issue (you are @copilot in this section)

@roblourens We wanted to rename this to `/new` for a long time except that it conflicted with `@workspace /new`

• Fixes /clear should archive current session and start a new one #285854

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[Copilot]

Pull request overview

This PR modifies the /clear slash command behavior to archive the current chat session before creating a new one, distinguishing it from passive session switching via the plus button.

Changes:

• The /clear command now archives the current session by calling setArchived(true) before executing ACTION_ID_NEW_CHAT
• Updated the command description from "Start a new chat" to "Start a new chat and archive the current one" to reflect the new behavior
• Injected IAgentSessionsService dependency to access session management functionality