Add autostash handling to update-dotfiles

[shunk031]

Summary

• Autostash dirty public/private chezmoi source changes before fetching and applying updates.
• Restore the autostash with git stash pop --index immediately after the update attempt so local work is not left hidden by default.
• Cover the happy path and restore-failure path in the update-dotfiles bats tests.

Autostash Risk Notes

• git stash pop --index can conflict after the source repository fast-forwards. When that happens, Git keeps the stash entry; the command now reports the restore failure and tells the operator to inspect the remaining stash.
• This only handles dirty source repositories. Destination-side chezmoi conflicts, such as files changed since chezmoi last wrote them, are separate and still require interactive handling or explicit skip behavior.

Validation

• bash -n home/dot_local/bin/exact_common/executable_update-dotfiles
• shfmt -i 4 -sr -d home/dot_local/bin/exact_common/executable_update-dotfiles tests/install/common/update_dotfiles.bats
• shellcheck home/dot_local/bin/exact_common/executable_update-dotfiles
• Manual stub smoke test for dirty-source autostash call order

Note: I did not run bats locally per the repository test policy; GitHub Actions should validate the bats suite.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.18%. Comparing base (3125d17) to head (5d4b5c3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #477      +/-   ##
==========================================
+ Coverage   90.19%   91.18%   +0.98%     
==========================================
  Files          10       10              
  Lines         306      329      +23     
==========================================
+ Hits          276      300      +24     
+ Misses         30       29       -1     

Flag	Coverage Δ
macos-14-client	41.66% <ø> (ø)
ubuntu-latest-client	91.15% <100.00%> (+1.65%)	⬆️
ubuntu-latest-server	90.54% <100.00%> (+0.35%)	⬆️

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