refactor(grizzly): Replace MatchGroup/context with Snippet and hit lists

[JeanMertz]

The SearchMatch type previously returned groups of context lines (similar to grep -C), which was unbounded in size and leaked raw note content into every search response.

Replace that model with a compact, size-bounded result:

• groups: Vec<MatchGroup> is removed. In its place, SearchMatch now carries line_hits (1-indexed line numbers, capped at max_line_hits), total_hits (always the true count), and a snippet — a short excerpt centered on the first match, truncated with … at snippet_chars characters.
• SearchMatch now also includes tags and updated_at, fetched in a single batch fetch_metadata query so callers no longer need a separate note_get round-trip just to identify a note.
• SearchParams drops the context field and gains snippet_chars (default 200) and max_line_hits (default 20).
• make_snippet is Unicode-safe: it counts chars, not bytes, and clamps the window to the actual match position.
• The MCP tool description for note_search is updated to explain the new response shape and how to follow up with note_get when full content is needed.
• Snippet is re-exported from the crate root.