feat: CLI Auto-updates

[RomneyDa]

Description

Adds configurable auto-updating to the CLI

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Summary by cubic

Adds configurable auto-updates to the Continue CLI with background checks on launch and a simple UI to run or skip updates. Addresses CON-2954.

• New Features

  • New UpdateService: checks latest version, compares, and installs updates (npm i -g @continuedev/cli).
  • Auto-update on start when GlobalContext.autoUpdateCli is true; prevents repeat updates in the same session.
  • New /update command opens an UpdateSelector in the TUI for manual updates.
  • CLI restarts itself after auto-update; manual updates prompt to restart.
  • Added update service/state types, service registration, and a minimal LoadingText component.
  • Navigation/UI wired to include the update selector screen.

• Migration

  • Auto-update is enabled by default. To disable: set GlobalContext.autoUpdateCli = false.