list: hint to use repo-list when no archive NAME is given Refs #9545

[charmikadi]

Description

When a user runs borg list without providing a required archive NAME,
they currently get a generic argparse error. This is confusing for users
migrating from borg1 who may expect borg list to list archives in a
repository (which is borg repo-list in borg2).

This change makes the name argument optional at the parser level and
adds an explicit, targeted error message in do_list that explains what
went wrong and suggests the correct command:

Error: borg list requires an archive NAME.
To list the archives in a repository, use: borg repo-list

Refs #9545

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.94%. Comparing base (106dfba) to head (1e21c2d).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9571   +/-   ##
=======================================
  Coverage   84.94%   84.94%           
=======================================
  Files          92       92           
  Lines       15291    15295    +4     
  Branches     2296     2297    +1     
=======================================
+ Hits        12989    12993    +4     
  Misses       1611     1611           
  Partials      691      691           

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[charmikadi]

Just bumping this PR for visibility. It implements issue #9545 (case 8) by adding a clear error message that suggests borg repo-list when borg list is run without an archive NAME.