feat(ps): expose per-storage-element common paths in --json output

[djgormley]

Composing download paths from ps --json requires re-deriving the common directory split that the files table computes internally (ps, pull, and create_files_table each inline it today). This adds a common_paths field to the --json payload: per storage element, the deepest common directory and the file names relative to it.

The derivation lives in utilities.common_paths(), and create_files_table now renders from the same helper -- which also fixes the single-file case, where the table previously showed the file itself as the common path with - . as the entry. An element with no usable split reports common_path: "" with the original paths, so nothing is hidden. Unit tests cover the split and no-split cases; the live ps --json test now asserts the field.

[tjzegmott]

Changes look good. Run pre-commit to pass CI.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.75000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.16%. Comparing base (a0646d7) to head (5ac42ce).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
dtcli/ps.py	50.00%	2 Missing ⚠️
tests/test_cli.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #176       +/-   ##
===========================================
- Coverage   67.01%   51.16%   -15.85%     
===========================================
  Files          15       17        +2     
  Lines        1640     1718       +78     
===========================================
- Hits         1099      879      -220     
- Misses        541      839      +298     

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[coveralls]

coverage: 51.164% (-15.8%) from 67.012% — WVURAIL:feat/ps-common-path into CHIMEFRB:main

[djgormley]

@tjzegmott -- Git is killing me. CI passes now after running precommit. Please re-approve.