docs(cli): document stack selection pattern semantics

[badmintoncryer]

Follow-up to #1909.

Adds a short paragraph to the README explaining that stack patterns are globs matched with OR semantics, so !A !B selects everything — use !(A|B) to exclude multiple stacks.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.27%. Comparing base (5766544) to head (4e19c19).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1912   +/-   ##
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  Coverage   91.27%   91.27%           
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  Files          79       79           
  Lines       12139    12139           
  Branches     1720     1720           
=======================================
  Hits        11080    11080           
  Misses       1024     1024           
  Partials       35       35           

Flag	Coverage Δ
suite.unit	91.27% <ø> (ø)

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[mrgrain]

Hey @badmintoncryer I think I have changed my mind on this. In this comment I have a minimal implementation that isn't depending on string matching.

#1909 (comment)

Would you still be interested in contributing this? Sorry about the back and forth.