Script to create a Changelog.md entry from files in .changelog

[j-rivero]

Follow up of #1335 , Part of #246.

The script consumes entries in the a .changelog/ directory containing files using a conventional commit text sentence and generates a Changelog.md modification and made a commit for it.

A testing run on gz-test would look like:

gz-test on  main [⇡] via △ v3.28.3 ❯ ~/code/release-tools2/source-repo-scripts/create_changelog_entry.py 
Reading changelog entries from /home/jrivero/code/gz/gz-test/.changelog...
Found commit SHA: 0da68529bda8739b2c7cef8231e063bb57e5ba3e for file my_feature1.md
Found PR #3 for commit 0da68529bda8739b2c7cef8231e063bb57e5ba3e
Found commit SHA: 26b1ab64abf66d57164d8bdb10a8e851827c88f1 for file update-license.md
Found PR #2 for commit 26b1ab64abf66d57164d8bdb10a8e851827c88f1
Generating changelog entry...
Calculated next version: 0.1.0
Generated changelog entry:
--------------------------------------------------
## Gazebo Test 0.1.0 (2026-02-18)

* feat: Support for the new feature 1 in all the components
      * [Pull request #3](https://github.com/gazebosim/gz-test/pull/3)
* fix: Update license years from 2012 to 2025
      * [Pull request #2](https://github.com/gazebosim/gz-test/pull/2)

--------------------------------------------------
Update Changelog.md with this entry? (y/N): 

Final result:

gz-test on  main [✘!⇡] via △ v3.28.3 ❯ git diff
diff --git a/Changelog.md b/Changelog.md
index 49190a0..ef47b15 100755
--- a/Changelog.md
+++ b/Changelog.md
@@ -1,3 +1,11 @@
 ## Gazebo Test 0.x
 
+## Gazebo Test 0.1.0 (2026-02-18)
+
+* feat: Support for the new feature 1 in all the components
+      * [Pull request #3](https://github.com/gazebosim/gz-test/pull/3)
+* fix: Update license years from 2012 to 2025
+      * [Pull request #2](https://github.com/gazebosim/gz-test/pull/2)
+
+
 ## Gazebo Test 0.0.0 (2022-01-01)
gz-test on  main [✘!⇡] via △ v3.28.3 ❯ git status
On branch main
Your branch is ahead of 'origin/main' by 2 commits.
  (use "git push" to publish your local commits)

Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
	deleted:    .changelog/my_feature1.md
	deleted:    .changelog/update-license.md

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   Changelog.md

TODO: add the commit feature