pr commenting: use native github CLI

[rugeli]

Problem

closes #422
documentation: https://cli.github.com/manual/gh_pr_comment

Solution

• removed iterative/setup-cml@v2
• updated both comment steps to use native gh cli commands
  • now uses an html comment marker to identify workflow-generated comments
• added logic: detect and delete if an existing comment found, post a fresh comment containing the latest report

Type of change

• New feature (non-breaking change which adds functionality)

Steps to Verify:

1. everything in the Comment job should behave the same as before
2. verify the comment is correctly post, updated, and only one exists at a time in this pr

[github-actions]

Packing analysis report

Analysis for packing results located at cellpack/tests/outputs/test_spheres/spheresSST

Ingredient name	Encapsulating radius	Average number packed
ext_A	25	236.0

Packing image

Distance analysis

Expected minimum distance: 50.00
Actual minimum distance: 50.01

Ingredient key	Pairwise distance distribution
ext_A

[mogres]

This looks great! Thanks for the update!