Consistent `Copilot CLI` entry point naming everywhere

[bamurtaugh]

As @DonJayamanne raised last week (and I noticed again in self-hosting and working on docs microsoft/vscode-docs#9443), we refer to the Copilot CLI differently in various places.

What @DonJayamanne identified:

• Copilot CLI => Terminal started from menu in + Sessions View
• New CLI Session => Menu to start a CLI session from menu in + Session View
• GitHub Copilot => Default title for all terminals when you run copilot in a terminal (controlled by CLI).
• GitHub Copilot CLI => Terminal profile dropdown menu near +
• Copilot CLI => Command group in command palette

I also found we have the commands Chat: New CLI Session and Chat: New CLI Session to the Side.

I think we're aligned that we want to stick with Copilot CLI everywhere above. cc @rebornix @lostintangent

[bamurtaugh]

Note I observed in self-hosting:

We have Chat: New Copilot CLI (chat-first) and Chat: New Copilot CLI Session (terminal-first). Should we rename them to Chat: New Copilot CLI Session (chat-first) and Chat: New Copilot CLI Terminal (terminal-first)?

[bamurtaugh]

Opened #300535 for the Claude Agent part.