fix: respect local config for autoupdate setting

[shubh73]

Why

autoupdate: false in local config gets ignored, only global config path is respected. Regression from opentui #2685 which recreated upgrade.ts with Config.global() instead of Config.get() and fix from #3408 was lost.

Fixes #6984.

How

Changed Config.global() to Config.get() so autoupdate reads from merged config.

Test Plan

Added tests for autoupdate config precedence.