Solves the universal "too many bookmarks, never read them" problem.

New commands:
- bookmarks sync: Archive bookmarks to local SQLite (survives tweet deletion)
- bookmarks list [--unread]: List recent or unread bookmarks
- bookmarks search "query": Full-text search across saved bookmarks
- bookmarks export [-o file.md] [--unread]: Export as markdown
- bookmarks digest [-d 7]: Weekly summary grouped by author
- bookmarks stats: Total, unread, top authors, date range

Key feature: sync archives the full tweet text locally. Even if the
original author deletes their tweet, your local copy survives. Search
is instant (SQLite), no API calls needed after initial sync.

Updated: README (bookmark section), SKILL.md (bookmark workflow),
agent-info (6 commands, bookmark_intelligence capability, usage hint).

28 tests passing. Zero compilation errors.