Sync "On a vacation" status to GitHub Busy status

[devinbinnie]

Adds an opt-in, per-user sync: when a user sets the "On a vacation" custom status (palm_tree emoji), their GitHub status is set to Busy so they aren't auto-assigned PRs. Setting any other custom status clears it.

• Off by default. Toggle with /github settings vacation-sync on|off (autocomplete and help updated). Turning it on while already on vacation applies Busy immediately; turning it off clears it.
• Triggered by the PreferencesHaveChanged hook: the server saves the "recent custom statuses" preference on every custom status set, and element 0 is the new status. Other preferences are ignored.
• A per-user KV flag records whether the plugin set Busy, so a GitHub status the user set themselves is never cleared.
• Uses GitHub's changeUserStatus GraphQL mutation (limitedAvailability).
• If GitHub rejects the update, the user gets a DM telling them to reconnect.

Known limitations:

• PreferencesHaveChanged does not fire when a custom status is removed or expires, so Busy is not cleared in those cases until another status is set or the user runs /github settings vacation-sync off.
• The mutation needs the OAuth user scope, which this PR adds to the requested scopes. Already-connected users must reconnect.
• Setting/clearing replaces any existing GitHub status the user had.

Tests: hook behavior (set, clear, ignore unrelated/disabled, GitHub failure) against an httptest GraphQL server, plus the settings command.