fix(release): do not end a changelog on a blank line

An entry carries a trailing blank line so that a `## [version]` below it is
separated. Two of the four placements in _insert_after_h1 put the entry at the
end of the file instead, where that separator becomes a blank line after the
final newline and markdownlint reports MD012. Both now pass the entry through
_at_end_of_file.

Only the first entry a file ever receives is affected, which is why it appears
on a repository's opening release and never again: every later entry is
inserted above an existing H2, and there the separator is correct. The
base-images repository hit it on 1.0.0 and its lint job went red on the first
push it ever had.

The other two placements are deliberately untouched. In the legacy branch the
old content follows the entry, and before the first H2 the previous release
section does, so the blank line has to stay.

Verified: four tests, one per placement, assert both that no document holds
three consecutive newlines and that it ends on exactly one; two of them also
assert the separator survives where it belongs, which fails if the trim is
applied too widely. 74 tests pass under tests/unit/pkgmgr/actions/release.

Unrelated and pre-existing: test_update_changelog_transforms_heading_and_inline_code
fails wherever markdownlint-cli2 is installed, because transform_changelog_message
renders a leading `#` as bold and markdownlint calls that MD036.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/pkgmgr/actions/release/files/changelog_md.py

@@ -16,6 +16,11 @@ H1_RE = re.compile(r"^#\s+\S", re.MULTILINE)
 H2_RE = re.compile(r"^##\s+\S", re.MULTILINE)
 
 
+def _at_end_of_file(entry: str) -> str:
+    """Return *entry* without the blank line that separates it from a next one."""
+    return entry.rstrip("\n") + "\n"
+
+
 def _insert_after_h1(existing: str, entry: str) -> str:
     """Place *entry* after the H1 (and any intro prose), above the first H2.
 
@@ -26,7 +31,7 @@ def _insert_after_h1(existing: str, entry: str) -> str:
     ``## ``) is preserved: *entry* is prepended unchanged.
     """
     if not existing.strip():
-        return f"# Changelog\n\n{entry}"
+        return f"# Changelog\n\n{_at_end_of_file(entry)}"
 
     if not H1_RE.search(existing):
         # Legacy layout: file starts with `## [version]` and has no H1.
@@ -43,7 +48,7 @@ def _insert_after_h1(existing: str, entry: str) -> str:
             if existing.endswith("\n\n")
             else ("\n" if existing.endswith("\n") else "\n\n")
         )
-        return f"{existing}{suffix}{entry}"
+        return f"{existing}{suffix}{_at_end_of_file(entry)}"
 
     # Insert new entry just before the first H2.
     head = existing[: h2_match.start()].rstrip("\n") + "\n\n"