Add support for preserving content before first H2 header as 00-preamble.md

[Copilot]

Problem

Previously, any content that appeared before the first H2 header (## Section) in a markdown file was silently ignored during the split operation. This meant that important introductory content like titles, metadata, or preamble text would be lost when splitting documents.

For example, given this markdown file:

# My Document Title

This is important introductory content that provides context.
It includes metadata and overview information.

## First Section
Content for first section...

## Second Section  
Content for second section...

The split operation would only create 01-first-section.md and 02-second-section.md, completely losing the title and introductory content.

Solution

This PR implements functionality to capture and preserve pre-H2 content in a special 00-preamble.md file:

• Content preservation: All content before the first H2 header is now saved as 00-preamble.md
• Smart filtering: Empty or whitespace-only preambles are ignored to avoid creating unnecessary files
• Backward compatibility: Files without preamble content work exactly as before
• Updated merge support: The merge command now handles 00- prefixed files correctly
• Round-trip integrity: Split → merge operations preserve the complete original document structure

Key Changes

1. Enhanced parsing logic in parseMarkdownSections() to collect pre-H2 content
2. Updated file creation to handle preamble sections with 00-preamble.md naming
3. Extended merge functionality to process files starting from 00- prefix
4. Updated documentation and help text to reflect the new capability

Testing

The implementation has been thoroughly tested with various scenarios:

• Documents with preamble + H2 sections → Creates 00-preamble.md plus numbered section files
• Documents with only preamble content → Creates single 00-preamble.md file
• Documents starting directly with H2 → Works unchanged (files start with 01-)
• Empty or whitespace-only files → Proper error handling
• Complex documents with subsections → All content preserved correctly
• Full round-trip (split → merge) → Original document structure maintained

This change ensures that no content is lost during markdown splitting operations while maintaining full backward compatibility with existing workflows.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.

[Copilot]

Pull Request Overview

This PR adds functionality to preserve content that appears before the first H2 header in markdown files by saving it to a special "00-preamble.md" file during the split operation.

• Modified parseMarkdownSections to capture pre-H2 content as a "preamble" section
• Updated split logic to create "00-preamble.md" files for content before the first H2 header
• Updated merge logic to process files starting from "00-" instead of "01-" to include preamble files

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cmd/split.go	Implements preamble content collection and creates "00-preamble.md" files with updated filename generation logic
cmd/merge.go	Updates merge logic to start processing from "00-" files and updates related error messages

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

The indexing logic is incorrect. When the first section is a preamble, non-preamble sections will have duplicate numbering. For example, if sections = [preamble, section1, section2], then section1 gets sectionNum = 1 (i=1, no increment) and section2 gets sectionNum = 2 (i=2, no increment), but this should be sectionNum = 1 for section1 and sectionNum = 2 for section2. The logic should decrement i when preamble exists: sectionNum = i for non-preamble sections when preamble is present.

See below for a potential fix:

	sectionCounter := 1
	for _, section := range sections {
		var filename string
		if section.title == "preamble" {
			filename = "00-preamble.md"
		} else {
			filename = fmt.Sprintf("%02d-%s.md", sectionCounter, sanitizeFilename(section.title))
			sectionCounter++

[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 5 changed files in this pull request and generated 2 comments.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

The section numbering logic is incorrect. When the first section is a preamble, sectionNum = i will result in section 1 being numbered as 01- (correct) but section 2 will be numbered as 01- instead of 02-. The logic should be sectionNum = i to skip the preamble index adjustment.

See below for a potential fix:

	sectionCount := 1
	for _, section := range sections {
		var filename string
		if section.title == "preamble" {
			filename = "00-preamble.md"
		} else {
			filename = fmt.Sprintf("%02d-%s.md", sectionCount, sanitizeFilename(section.title))
			sectionCount++

[Copilot]

Pull Request Overview

Copilot reviewed 2 out of 5 changed files in this pull request and generated 2 comments.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}