Three robot-mode print functions used `serde_json::to_string().unwrap_or_default()`
which silently outputs an empty string on failure (exit 0, no error). This
diverged from the codebase standard in handlers.rs which uses `?` propagation.
Changed to return Result<()> with proper LoreError::Other mapping:
- explain.rs: print_explain_json()
- file_history.rs: print_file_history_json()
- trace.rs: print_trace_json()
Updated callers in handlers.rs and explain.rs to propagate with `?`.
While serde_json::to_string on a json!() Value is unlikely to fail in practice
(only non-finite floats trigger it), the unwrap_or_default pattern violates the
robot mode contract: callers expect either valid JSON on stdout or a structured
error on stderr with a non-zero exit code, never empty output with exit 0.
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>
