Guide users to sign in from more extension entry points

[alexr00]

Extension entry points:

• Views: already have a sign in button
• Commands: most commands are hidden from the command palette when not signed in
  • ❓ Are there some that we should enabled?
• "Delegate to coding agent": currently hidden when not signed in
  • ❗We should show this and then prompt the user to sign in
• @githubpr: currently hidden when not signed in
  • ❗We should show this and then prompt the user to sign in

[alexr00]

Some additional places from @bamurtaugh:

• #copilotCodingAgent
• Chat Sessions view