Problem

When a CHANGELOG.md file starts directly with a version heading (e.g. ## 1.3.0) rather than a package title heading (e.g. # my-package), newly generated version entries are inserted after the first line (the existing version heading), not before it.

This produces a changelog like:

## 1.3.0

## 1.4.0      <- new entry wrongly follows the old heading

### Minor Changes

- New feature

### Minor Changes  <- old 1.3.0 content now appears under 1.4.0

- Old feature

This is the bug reported in #1056. The Andarist comment in that issue suggests detecting whether the top heading matches a version, which is what this PR does.

Root Cause

In packages/apply-release-plan/src/index.ts, the prependFile function always inserts new content after the first line of the file. This works when the first line is a package title (# my-package), but breaks when the first line is already a version heading.

Fix

The prependFile function now detects whether the first line is a version heading (using /^#{1,6}\s+\d+\.\d+/). When it is, the new changelog entry is prepended before the entire existing content:

const firstLine = fileData.split('\n')[0];
const isVersionHeading = /^#{1,6}\s+\d+\.\d+/.test(firstLine);

if (isVersionHeading) {
  // File starts with a version heading (no package title) - prepend before everything
  newChangelog = data.trimStart() + '\n\n' + fileData;
} else {
  // File starts with a package title - insert after first line (existing behavior)
  const index = fileData.indexOf('\n');
  newChangelog = index === -1 ? fileData + data : fileData.slice(0, index) + data + fileData.slice(index + 1);
}

Test

Added a test that:

1. Creates a CHANGELOG.md starting with ## 1.0.0 (no package title)
2. Applies a release plan that bumps from 1.0.0 to 1.1.0
3. Asserts that ## 1.1.0 appears before ## 1.0.0 in the output

Fixes #1056