Rename background session context menu items from "Worktree" to "Session"

[Copilot]

Background sessions can now be created via Workspace (non-git folders), making "Worktree" misleading in the context menu. Updates the three affected NLS strings to use "Session" instead.

Changes

• package.nls.json: Updated display strings for the three session context menu items:
  • "Open Worktree in New Window" → "Open Session in New Window"
  • "Open Worktree in Integrated Terminal" → "Open Session in Integrated Terminal"
  • "Resume in Integrated Terminal" → "Resume Session in Integrated Terminal"

Command IDs (openWorktreeInNewWindow, openWorktreeInTerminal) are unchanged. "Commit File to Worktree" is intentionally left as-is since it refers to a specific git operation.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Change Menu Terminology from 'Worktree' to 'Session' for Background (CopilotCLI) Sessions</issue_title>
<issue_description>With the new option allowing background sessions to be created using either Worktree or Workspace, the current menu options referring specifically to 'Worktree' are no longer universally appropriate. If a user starts a background session in a non-git folder or via Workspace options, the term 'worktree' can be confusing and misleading.

Proposed Improvement:
Update the menu options to use the term 'Session' for better clarity and universality. Suggested new options:

• Open Session in New Window
• Open Session in Integrated Terminal
• Resume Session in Integrated Terminal

This change will make the options clearer in all scenarios, regardless of the underlying method for background session creation.

</issue_description>

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

• Fixes Change Menu Terminology from 'Worktree' to 'Session' for Background (CopilotCLI) Sessions vscode#298170

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