GitHub access checks now distinguish a 'not connected' account from other failures

What

• GitHub App / linked-account access checks — used for Cloud sessions and background-task remote eligibility — now distinguish the case where the user's linked account simply isn't connected to GitHub at all, surfacing a specific github_not_connected reason/type instead of lumping it in with transient failures.
• The routine-setup GitHub access check now returns a structured {hasAccess, reason} result instead of a plain boolean, with its own message for the github_not_connected case (separate from the generic "run /web-setup" message).
• Cloud session bootstrap (teleportToRemote) now checks for this same github_not_connected state during its preflight checks and shows a dedicated failure message instead of the generic "app not installed" one. (This same code path also became better at picking a starting branch/commit when the local branch has diverged from, or shares no history with, the remote.)
• The github_not_connected state is threaded through to the background-task remote-eligibility check and the cloud-access-reason UI, which shows a dedicated message pointing users to connect their account on the web or run /web-setup.

Why Previously, not having a GitHub account linked could look like the same generic error as a temporary connection problem. Now Claude Code tells you specifically that your account isn't connected and points you to fix it, rather than leaving you to guess.