fix(vale): scope changed-files diff to PR branch additions only

scripts/vale/changed_lines_to_json.py

@@ -29,7 +29,11 @@
 def get_changed_files(base_sha, head_sha, pattern=r'^docs/.*\.(md|mdx)$'):
     """Get list of changed files matching the pattern."""
     try:
-        cmd = f"git diff --name-only {base_sha} {head_sha}"
+        # Three-dot diff (base...head) scopes the comparison to the changes
+        # the PR branch added since it diverged from base. Two-dot or
+        # endpoint form would also surface commits main has gained since
+        # the branch was created, causing Vale to run on unrelated files.
+        cmd = f"git diff --name-only {base_sha}...{head_sha}"
         result = subprocess.check_output(cmd, shell=True, text=True)
         all_files = result.splitlines()
 
@@ -45,7 +49,7 @@ def get_changed_files(base_sha, head_sha, pattern=r'^docs/.*\.(md|mdx)$'):
 def get_changed_lines(file_path, base_sha, head_sha):
     """Get line numbers that were changed in a specific file."""
     try:
-        cmd = f"git diff --unified=0 {base_sha} {head_sha} -- {file_path}"
+        cmd = f"git diff --unified=0 {base_sha}...{head_sha} -- {file_path}"
         diff_output = subprocess.check_output(cmd, shell=True, text=True)
 
         changed_lines = []