feat: add a setting to turn off source control

People who manage Git themselves can now stop T3 Code from reading their
repositories. With sourceControlEnabled off, the server treats every folder
as having no repository: no checkpoint refs after turns, no Git status,
diffs, branch or pull request lookups, and no automatic pull or worktree
cleanup. The switch is under Settings > Source Control on web and mobile
and applies right away.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
141ad94353Wills Manley committed on 10/22/2026, 9:28:10 PMparent2c8bdf6

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