Default background agent repo to SCM-selected repo in multi-root workspaces

[Copilot]

In multi-root workspaces, new background agent sessions defaulted to the MRU or first available repo instead of the repo currently selected in the SCM view, requiring a manual correction each time.

Change

• getDefaultUntitledSessionRepositoryOption (copilotCLIChatSessionsContribution.ts): When no folder is already stored for the session, use gitService.activeRepository (the SCM-selected repo) as the first preference, before falling back to MRU then first available.

New priority order for the default repo picker:

1. Previously stored session folder (unchanged)
2. SCM-active repository ← new
3. Last used folder (MRU)
4. First available repository

const activeRepo = this.gitService.activeRepository.get();
const activeRepoId = activeRepo?.rootUri.fsPath;
const lastUsedFolderId = this.folderRepositoryManager.getLastUsedFolderIdInUntitledWorkspace();
const firstRepo = (activeRepoId && repositories.find(repo => repo.id === activeRepoId)?.id)
    ?? (lastUsedFolderId && repositories.find(repo => repo.id === lastUsedFolderId)?.id)
    ?? repositories[0].id;

Original prompt

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This section details on the original issue you should resolve

<issue_title>Worktree repo doesn't align with scm multiroot repo</issue_title>
<issue_description>🩸 Picked repo for background agents should align with the scm selected repo in multi root workflows. Changing it manually each time is a bit of a papercut

<agent_instructions>The changes are to be made in src/extension/chatSessions/vscode-node/copilotCLIChatSessionsContribution.ts
See getDefaultUntitledSessionRepositoryOption method</agent_instructions>

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

@DonJayamanne Agreed.

• Fixes Worktree repo doesn't align with scm multiroot repo vscode#297432

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