ai: fix UTF-8 byte-slice panics in insight_generator log/truncation paths

Switch four `&s[..N]` / `&s[..s.len().min(N)]` sites to
`chars().take(N).collect::<String>()` so truncation lands on character
boundaries instead of mid-codepoint. The agentic summary preview log
was panicking when generated content hit an em-dash at byte 200; the
few-shot passage cap, brief_json_args debug formatter, and a test
assertion message had the same latent bug.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ai/insight_generator.rs

@@ -1749,8 +1749,8 @@ Return ONLY the summary, nothing else."#,
             .iter()
             .enumerate()
             .map(|(i, c)| {
-                let trimmed = if c.len() > 1000 {
+                let trimmed = if c.chars().count() > 1000 {
-                    format!("{}…", &c[..1000])
+                    format!("{}…", c.chars().take(1000).collect::<String>())
                 } else {
                     c.clone()
                 };
@@ -3406,8 +3406,8 @@ Return ONLY the summary, nothing else."#,
         obj.iter()
             .map(|(k, v)| {
                 let rendered = match v {
-                    serde_json::Value::String(s) if s.len() > 40 => {
+                    serde_json::Value::String(s) if s.chars().count() > 40 => {
-                        format!("\"{}...\"", &s[..40])
+                        format!("\"{}...\"", s.chars().take(40).collect::<String>())
                     }
                     _ => v.to_string(),
                 };
@@ -4088,10 +4088,11 @@ Return ONLY the summary, nothing else."#,
         let title = title_raw.trim().trim_matches('"').to_string();
 
         log::info!("Agentic generated title: {}", title);
+        let summary_preview: String = final_content.chars().take(200).collect();
         log::info!(
             "Agentic generated summary ({} chars): {}",
             final_content.len(),
-            &final_content[..final_content.len().min(200)]
+            summary_preview
         );
 
         // 14. Serialize the full message history for training data
@@ -4671,7 +4672,7 @@ mod tests {
         assert!(
             out.starts_with("You are a journal writer in first person, warm and reflective."),
             "custom prompt must lead the system content; got: {}",
-            &out[..out.len().min(200)],
+            out.chars().take(200).collect::<String>(),
         );
         assert!(
             !out.contains("personal photo memory assistant"),