feat(gutenberg): add book browsing commands

[jiejia]

Gutenberg adapter

Adds a public HTML-backed Project Gutenberg adapter with nine commands:

Command	Purpose
ebooks <id>	Show ebook metadata and download URLs for all available formats
search [title]	Search ebooks by title or keyword with optional sorting and limit
author <id>	List all books by a Gutenberg author
hot-books	Show the top book rankings for yesterday, 7 days, or 30 days
hot-author	Show the top author rankings for yesterday, 7 days, or 30 days
main-categories	List the main Gutenberg book categories and their parent categories
collections	List collections from the Gutenberg Reading Lists page
categories	List Reading List categories, optionally filtered by initial letter
category-search <query>	Search bookshelf categories with title, downloads, quantity, or release date sorting

Contract and strategy

• Public reads use server-rendered Gutenberg HTML and declare Strategy.PUBLIC.
• The adapter does not require authentication, browser sessions, or private credentials.
• Shared request, parsing, pagination, validation, and error-handling logic is implemented in utils.js.
• The adapter returns stable identifiers, names, URLs, authors, download counts, and ebook format links.
• OpenCLI's title sort option is mapped to Gutenberg's alpha sort parameter.
• category-search merges download-sorted results when the selected sort mode does not expose download counts directly.
• Gutenberg pagination links are followed and duplicate results are removed.
• collections preserves Gutenberg's source ordering.
• author supports title, downloads, and release_date sorting.
• category-search supports title, downloads, quantity, and release_date sorting.
• Download counts are normalized to numeric values where available.
• Empty results, malformed rows, and missing download formats are reported through typed command errors.
• HTML attributes and entities are decoded consistently across the supported page types.

Verification

• Focused Gutenberg tests: 9 files / 28 tests pass.
• npm run typecheck: pass.
• npm run build: pass; manifest compiled 1,375 entries.
• opencli validate gutenberg: 0 errors, 0 warnings.
• npm run check:typed-error-lint: pass with no new violations.
• npm run check:silent-column-drop: pass with no new violations.
• git diff --check: pass.