fix: prevent panics in robot-mode JSON output and arithmetic paths

Peer code review found multiple panic-reachable paths:

1. serde_json::to_string().unwrap() in 4 robot-mode output functions
   (who.rs, main.rs x3). If serialization ever failed (e.g., NaN from
   edge-case division), the CLI would panic with an unhelpful stack trace.
   Replaced with unwrap_or_else that emits a structured JSON error fallback.

2. encode_rowid() in chunk_ids.rs used unchecked multiplication
   (document_id * 1000). On extreme document IDs this could silently wrap
   in release mode, causing embedding rowid collisions. Now uses
   checked_mul + checked_add with a diagnostic panic message.

3. HTTP response body truncation at byte index 500 in client.rs could
   split a multi-byte UTF-8 character, causing a panic. Now uses
   floor_char_boundary(500) for safe truncation.

4. who.rs reviews mode: SQL used `m.author_username != ?1` which silently
   dropped MRs with NULL author_username (SQL NULL != anything = NULL).
   Changed to `(m.author_username IS NULL OR m.author_username != ?1)`
   to match the pattern already used in expert mode.

5. handle_auth_test hardcoded exit code 5 for all errors regardless of
   type. Config not found (20), token not set (4), and network errors (8)
   all incorrectly returned 5. Now uses e.exit_code() from the actual
   LoreError, with proper suggestion hints in human mode.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/commands/who.rs

@@ -851,7 +851,7 @@ fn query_reviews(
          WHERE n.author_username = ?1
            AND n.note_type = 'DiffNote'
            AND n.is_system = 0
-           AND m.author_username != ?1
+           AND (m.author_username IS NULL OR m.author_username != ?1)
            AND n.created_at >= ?2
            AND (?3 IS NULL OR n.project_id = ?3)";
 
@@ -868,7 +868,7 @@ fn query_reviews(
          WHERE n.author_username = ?1
            AND n.note_type = 'DiffNote'
            AND n.is_system = 0
-           AND m.author_username != ?1
+           AND (m.author_username IS NULL OR m.author_username != ?1)
            AND n.created_at >= ?2
            AND (?3 IS NULL OR n.project_id = ?3)";
 
@@ -888,7 +888,7 @@ fn query_reviews(
          WHERE n.author_username = ?1
            AND n.note_type = 'DiffNote'
            AND n.is_system = 0
-           AND m.author_username != ?1
+           AND (m.author_username IS NULL OR m.author_username != ?1)
            AND ltrim(n.body) LIKE '**%**%'
            AND n.created_at >= ?2
            AND (?3 IS NULL OR n.project_id = ?3)
@@ -1798,7 +1798,12 @@ pub fn print_who_json(run: &WhoRun, args: &WhoArgs, elapsed_ms: u64) {
         meta: RobotMeta { elapsed_ms },
     };
 
-    println!("{}", serde_json::to_string(&output).unwrap());
+    println!(
+        "{}",
+        serde_json::to_string(&output).unwrap_or_else(|e| {
+            format!(r#"{{"ok":false,"error":{{"code":"INTERNAL_ERROR","message":"JSON serialization failed: {e}"}}}}"#)
+        })
+    );
 }
 
 #[derive(Serialize)]