Show the actual default config directory in --help

[mbrt]

Issue #463 reports that gmailctl init always uses ~/.gmailctl and ignores the XDG config directory. That was true in v0.12.0, the version it was tested with, but the XDG support landed afterwards in #455 and is not released yet.

What remained wrong was the documentation, which made the location hard to guess:

• The --config help text claimed the fallback is always $HOME/.config/gmailctl.
• The README referred to <XDG_BASE_DIR>, which is not a variable defined by the XDG spec.

The help text now prints the resolved path, computed the same way as the directory actually used, and the README names XDG_CONFIG_HOME, its per-platform defaults, and points to --help for the exact path.

Close #463.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.22%. Comparing base (3bb877d) to head (14a4125).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/gmailctl/cmd/root_cmd.go	0.00%	3 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) 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     #466      +/-   ##
==========================================
- Coverage   39.23%   39.22%   -0.02%     
==========================================
  Files          54       54              
  Lines        3680     3681       +1     
==========================================
  Hits         1444     1444              
- Misses       2135     2136       +1     
  Partials      101      101              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.