Add --include-labels flag to the download command

[ronnie]

Summary

gmailctl download always emits a labels: section (plus a comment explaining that labels management is optional and can be removed by hand). Users who prefer to keep managing labels in the Gmail UI currently have to delete that section manually after every download.

This adds an --include-labels flag to download, defaulting to true. With --include-labels=false, the downloaded labels are dropped before the config is built, so the generated Jsonnet has neither the labels: section nor its comment. Because labels management is opt-in based on the presence of labels in the config, diff, apply and edit then leave labels alone with no further changes.

• New --include-labels flag on download (default true, existing behaviour unchanged).
• One-sentence README note in the "Migrate from another solution" section.

Test plan

• go test ./... passes.
• go build ./cmd/gmailctl && ./gmailctl download --help shows the new flag.
• Manually verified against a real account: default output and --include-labels=false output differ only by the labels: section and its comment, and diff ignores a label added in the Gmail UI only for the config downloaded without labels.

[mbrt]

Thanks @ronnie!

[codecov]

Codecov Report

❌ Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.24%. Comparing base (9cf4f0b) to head (51632ea).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/gmailctl/cmd/download_cmd.go	14.28%	6 Missing ⚠️

❌ Your patch check has failed because the patch coverage (14.28%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #470      +/-   ##
==========================================
+ Coverage   41.94%   42.24%   +0.30%     
==========================================
  Files          54       54              
  Lines        3710     3728      +18     
==========================================
+ Hits         1556     1575      +19     
+ Misses       2053     2052       -1     
  Partials      101      101              

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