Add SQLite index and mark search

[polymorcodeus]

Summary

Adds a derived SQLite index over the TOML shelf files so reads and searches hit the index instead of a full TOML parse, and a book mark search command with FTS5 full-text search over titles and URLs plus tag, shelf, and collection filters.

What's new

• Derived SQLite index - a gitignored index (modernc.org/sqlite, pure Go) rebuilt from TOML; shelf list, collection list, and mark list now read from it while writes stay TOML-first.
• book mark search - FTS5 search over title and URL, non-interactive, with plain-text output (title and URL per line) by default.
• Tag filtering - --tags uses comma for OR and plus for AND (a,b+c means (a OR b) AND c); --shelf and --collection scope results.
• Index management - book index rebuild and book index sync recover from corruption and reconcile external edits.

Usage

Command	Purpose
book mark search <query>	full-text search over title and URL
book mark search --tags a,b+c	filter by tags (OR within, AND across)
book mark search --shelf X --collection Y	scope results to a shelf and collection
book index rebuild / book index sync	rebuild or reconcile the index

Behavior changes / guardrails

• TOML remains the git-committed source of truth; the index is derived and disposable, always rebuildable.
• Reads reconcile the index with a stat fast path, hashing only when mtime or size changes, so external edits (git pull, manual changes) are picked up without a full rebuild.
• book mark search is non-interactive; running it with no query and no filters errors with guidance.
• Tag filters reject empty groups (a,, ,a, a+, +a).
• Soft-deleted marks (deleted_at) are excluded from search results.
• The index lives in $XDG_CACHE_HOME/book/index.db, falling back to the config dir (with a .gitignore write) when the cache dir is unavailable.

Testing

• Unit tests cover the index schema, rebuild, incremental sync (including prune and unchanged-touch), read queries, tag grammar, tag-filtered search, and soft-delete exclusion.
• make check runs fmt, vet, lint, and tests; all green.