fix(cli): handle multibyte commands in list output

[shunichironomura]

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Summary

• truncate listed commands by Unicode character count instead of UTF-8 byte offsets
• add a CLI regression test covering a multibyte command that previously panicked

Closes #1068

AI assistance

• AI used: yes — Amp investigated the issue, implemented the fix, and ran verification
• Human review of AI-assisted code: pending

Breaking changes

• No breaking changes
• Breaking changes; the breaking change label is applied

[codecov-commenter]

Codecov Report

❌ Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.11%. Comparing base (433b438) to head (2e8c0af).

Files with missing lines	Patch %	Lines
crates/capsula-cli/src/main.rs	16.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1223      +/-   ##
==========================================
- Coverage   54.16%   54.11%   -0.06%     
==========================================
  Files          44       44              
  Lines        4233     4237       +4     
==========================================
  Hits         2293     2293              
- Misses       1940     1944       +4     

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