feat(trace,file-history): add tracing instrumentation and diagnostic hints

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>

src/cli/commands/trace.rs

@@ -50,17 +50,23 @@ pub fn print_trace(result: &TraceResult) {
         );
     }
 
+    // Show searched paths when there are renames but no chains
     if result.trace_chains.is_empty() {
         println!(
             "\n  {} {}",
             Icons::info(),
             Theme::dim().render("No trace chains found for this file.")
         );
-        println!(
-            "  {}",
-            Theme::dim()
-                .render("Hint: Run 'lore sync' to fetch MR file changes and cross-references.")
-        );
+        if !result.renames_followed && result.resolved_paths.len() == 1 {
+            println!(
+                "  {} Searched: {}",
+                Icons::info(),
+                Theme::dim().render(&result.resolved_paths[0])
+            );
+        }
+        for hint in &result.hints {
+            println!("  {} {}", Icons::info(), Theme::dim().render(hint));
+        }
         println!();
         return;
     }
@@ -195,6 +201,7 @@ pub fn print_trace_json(result: &TraceResult, elapsed_ms: u64, line_requested: O
             "elapsed_ms": elapsed_ms,
             "total_chains": result.total_chains,
             "renames_followed": result.renames_followed,
+            "hints": if result.hints.is_empty() { None } else { Some(&result.hints) },
         }
     });