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/embedding/chunk_ids.rs

@@ -1,7 +1,12 @@
 pub const CHUNK_ROWID_MULTIPLIER: i64 = 1000;
 
 pub fn encode_rowid(document_id: i64, chunk_index: i64) -> i64 {
-    document_id * CHUNK_ROWID_MULTIPLIER + chunk_index
+    document_id
+        .checked_mul(CHUNK_ROWID_MULTIPLIER)
+        .and_then(|v| v.checked_add(chunk_index))
+        .unwrap_or_else(|| {
+            panic!("encode_rowid overflow: document_id={document_id}, chunk_index={chunk_index}")
+        })
 }
 
 pub fn decode_rowid(rowid: i64) -> (i64, i64) {