Add structured tracing spans to trace and file-history pipelines so debug
logging (-vv) shows path resolution counts, MR match counts, and discussion
counts at each stage. This makes empty-result debugging straightforward.
Add a hints field to TraceResult and FileHistoryResult that carries
machine-readable diagnostic strings explaining *why* results may be empty
(e.g., "Run 'lore sync' to fetch MR file changes"). The CLI renders these
as info lines; robot mode includes them in JSON when non-empty.
Also: fix filter_map(Result::ok) → collect::<Result> in trace.rs (same
pattern fixed in prior commit for file_history/path_resolver), and switch
conn.prepare → conn.prepare_cached for the MR query.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
