fix(robot): propagate JSON serialization errors instead of silent failure

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.

src/cli/commands/file_history.rs

@@ -5,7 +5,7 @@ use tracing::info;
 use crate::Config;
 use crate::cli::render::{self, Icons, Theme};
 use crate::core::db::create_connection;
-use crate::core::error::Result;
+use crate::core::error::{LoreError, Result};
 use crate::core::file_history::resolve_rename_chain;
 use crate::core::paths::get_db_path;
 use crate::core::project::resolve_project;
@@ -391,7 +391,7 @@ pub fn print_file_history(result: &FileHistoryResult) {
 
 // ── Robot (JSON) output ─────────────────────────────────────────────────────
 
-pub fn print_file_history_json(result: &FileHistoryResult, elapsed_ms: u64) {
+pub fn print_file_history_json(result: &FileHistoryResult, elapsed_ms: u64) -> Result<()> {
     let output = serde_json::json!({
         "ok": true,
         "data": {
@@ -409,5 +409,10 @@ pub fn print_file_history_json(result: &FileHistoryResult, elapsed_ms: u64) {
         }
     });
 
-    println!("{}", serde_json::to_string(&output).unwrap_or_default());
+    println!(
+        "{}",
+        serde_json::to_string(&output)
+            .map_err(|e| LoreError::Other(format!("JSON serialization failed: {e}")))?
+    );
+    Ok(())
 }

src/cli/commands/trace.rs

@@ -1,4 +1,5 @@
 use crate::cli::render::{Icons, Theme};
+use crate::core::error::{LoreError, Result};
 use crate::core::trace::{TraceChain, TraceResult};
 
 /// Parse a path with optional `:line` suffix.
@@ -152,7 +153,11 @@ fn truncate_body(body: &str, max: usize) -> String {
     format!("{}...", &body[..boundary])
 }
 
-pub fn print_trace_json(result: &TraceResult, elapsed_ms: u64, line_requested: Option<u32>) {
+pub fn print_trace_json(
+    result: &TraceResult,
+    elapsed_ms: u64,
+    line_requested: Option<u32>,
+) -> Result<()> {
     // Truncate discussion bodies for token efficiency in robot mode
     let chains: Vec<serde_json::Value> = result
         .trace_chains
@@ -205,7 +210,12 @@ pub fn print_trace_json(result: &TraceResult, elapsed_ms: u64, line_requested: O
         }
     });
 
-    println!("{}", serde_json::to_string(&output).unwrap_or_default());
+    println!(
+        "{}",
+        serde_json::to_string(&output)
+            .map_err(|e| LoreError::Other(format!("JSON serialization failed: {e}")))?
+    );
+    Ok(())
 }
 
 #[cfg(test)]