Backlinks Tracking Step

## Context

Users want to see which pages link to the current page (backlinks). This requires tracking all outgoing links and computing the reverse mapping.

## Requirements

1. Create a step that runs after markdown rendering
2. Parse HTML to extract all internal links
3. Build a map of URL → pages that link to it
4. Store in pipeline context for template access

## Acceptance Criteria

- [ ] Each content object has `backlinks` array after step runs
- [ ] Each content object has `outgoingLinks` array
- [ ] Backlinks contain URL and title of linking page
- [ ] Only internal links are tracked (not external URLs)
- [ ] Relative links are resolved correctly
- [ ] Anchor fragments are stripped from URLs

## Test Cases

1. Page A links to Page B → Page B's backlinks includes Page A
2. Page A links to Page B twice → Page B's backlinks includes Page A once
3. Page A links to external site → Not included in backlinks

## Notes

This step needs to parse rendered HTML, so it must run after MarkdownRenderStep. Consider saving the hast html AST from the markdown step, to avoid parsing twice.