docs(CHANGES): Verbatim command output by default

why: Document the verbatim-output breaking change and the related
output-fidelity fixes for downstream readers.

what:
- Add a Breaking changes entry for the verbatim-default `.run()`
  (with the trim=True migration) and Fixes for applyable diffs,
  byte-identical blobs, and intact multi-line error output.

Author: tony

CHANGES

@@ -20,6 +20,31 @@ $ uv add libvcs --prerelease allow
 _Notes on the upcoming release will go here._
 <!-- END PLACEHOLDER - ADD NEW CHANGELOG ENTRIES BELOW THIS LINE -->
 
+### Breaking changes
+
+#### Command output is returned verbatim by default (#538)
+
+`.run()` on {class}`~libvcs.cmd.git.Git`, {class}`~libvcs.cmd.hg.Hg`, and {class}`~libvcs.cmd.svn.Svn` no longer strips each line, drops blank lines, or removes the trailing newline — it returns the output unchanged.
+
+Before, a bare value came back trimmed:
+
+```python
+sha = git.run(["rev-parse", "HEAD"])
+```
+
+After, output is verbatim; pass `trim=True` for the previous behavior:
+
+```python
+sha = git.run(["rev-parse", "HEAD"], trim=True)
+```
+
+High-level helpers are unaffected: {meth}`~libvcs.sync.git.GitSync.get_revision` still returns a bare revision.
+
+### Fixes
+
+- A captured `git diff` re-applies with `git apply` and `git cat-file blob` is byte-identical — leading indentation, blank lines, and the trailing newline are preserved instead of stripped (#538).
+- Multi-line stderr in {exc}`~libvcs.exc.CommandError` keeps its line breaks instead of being concatenated into one run-on line (#538).
+
 ## libvcs 0.43.0 (2026-06-20)
 
 libvcs 0.43.0 restores compatibility with pytest 9.1. Because the pytest plugin is auto-loaded by pytest, the previous release aborted the test session for any project that had libvcs installed and upgraded to pytest 9.1+; that no longer happens. Downstream tools such as vcspull are the primary beneficiaries.