docs: use archive series names instead of borg 1.x style names

[ThomasWaldmann]

Removes remaining doc examples that put the hostname, user or a date into the archive name (borg 1.x style). borg 2 stores host, user and creation time in the archive metadata, and a stable archive name per series is needed for files cache reuse and for prune.

• create: drop the '{hostname}-{user}' archive name example.
• delete, prune: use host:{hostname} / date: match selectors instead of sh:{hostname}-* and date-in-name globs; prune a named series (backup-sys-home).
• copy, list, tar: use series names (home, root, docs) instead of dated or weekday names.
• mount: borgfs example output now shows the default name-shortid directory names.
• completion: docstring example archive name without a year.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.34%. Comparing base (3c95ce7) to head (7ed9b55).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10528      +/-   ##
==========================================
+ Coverage   89.32%   89.34%   +0.01%     
==========================================
  Files         104      104              
  Lines       20079    20079              
  Branches     3152     3152              
==========================================
+ Hits        17936    17940       +4     
+ Misses       1484     1481       -3     
+ Partials      659      658       -1     

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