Published/Draft Filtering

**Dependencies**: None

## Context

Users need to mark pages as drafts that shouldn't appear in production builds. The `published` frontmatter field should control this.

## Requirements

1. Add filtering logic to exclude `published: false` pages

## Acceptance Criteria

- [ ] Pages with `published: false` are excluded from build
- [ ] Pages with `published: true` are included
- [ ] Pages without `published` field are excluded (default behavior)
- [ ] `archavist build --drafts` includes all pages
- [ ] Console shows count of filtered pages

## Test Cases

```yaml
# Should be excluded (production)
---
title: Work in Progress
published: false
---

# Should be included
---
title: Finished Article
published: true
---

# Should be included (no field = published)
---
title: Regular Page
---
```