Keep PR maintenance tests off the global git config and scope the Chat unread transition check

Two test failures fixed:

- scripts/pr-maintenance tests and the code they exercise run git
  with the developer's global and system config. With
  commit.gpgsign=true (SSH format, no signing key) every fixture
  commit fails, so 47 tests fail on such machines. The five test
  files that run git now import isolated-git.test-fixtures, which
  points GIT_CONFIG_GLOBAL at the null device and sets
  GIT_CONFIG_NOSYSTEM.
- "a closed desktop Chat tab keeps unread activity visible" counted
  transitions on every [data-motion-feedback="count"] on the page.
  Since the Projects sidebar decision counts (98a8bffd..87d6bdcb), the
  sidebar's quiet Count uses the same feedback, and it is keyed to
  replay its pop when a count rises, so a sidebar badge is replaced
  mid-transition. Its transitionend never fires, and ends >= starts
  can't hold. The test now counts only the Chat toggle's badge. CI on
  main has failed this test since 87d6bdcb.

Assistant-workflow: inline
Assistant-model: Claude Opus 5.5 (fast)
Assistant-verification: bun test passed: 4407 pass, 3 skips, 0 fail with commit.gpgsign=true still in the global config; 47 failed on main with the same config
Assistant-verification: playwright e2e passed: responsive-activity.e2e.ts 9/9 three runs in a row; it failed on main locally and in CI run 37855394975
Assistant-verification: manual diagnosis passed: a MutationObserver in the test showed the transitioning count badges removed mid-transition were span.project-sidebar-count children, not the Chat toggle
Assistant-verification: bun run types passed; bun run ci passed: no new findings
Co-authored-by: Alex Lavaee <lavaman131@github.com>