feat: zstash ls --tars filters to tars containing matched files

[Copilot]

Summary

Previously zstash ls --tars always dumped every tar in the archive regardless of any file filter. Users wanting to download a subset of the archive (e.g. "years 1850–1900 from the historical run") had no way to identify which specific tars they needed.

Objectives:

• Make --tars show only the tars that contain the files matched by the [files] glob pattern
• Add unit tests covering filtered and unfiltered cases
• Document the workflow with an end-to-end example

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version
• an incompatible (non-backwards compatible) API change: increment the major version

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a single commit.
• Logic: I have visually inspected the entire pull request myself.
• Pre-commit checks: All the pre-commits checks have passed.

[forsyth2]

Alternative approach that doesn't involve zstash code change:

# Find the db file zstash is using (see get_db_filename() in settings.py for the exact path;
# typically something like <cache>/index.db)
DB=zstash/index.db

sqlite3 -header -column "$DB" "
  SELECT DISTINCT tars.*
  FROM tars
  JOIN files ON files.tar = tars.name
  WHERE files.name GLOB '*historical*'
  ORDER BY tars.name;
"