Summary error (vibe-kanban) (#530)

* ## Summary

Fixed the string slicing panic in [crates/local-deployment/src/container.rs](file:///private/var/folders/m1/9q_ct1913z10v6wbnv54j25r0000gn/T/vibe-kanban/worktrees/vk-0ec9-summary-er/crates/local-deployment/src/container.rs#L1106-L1118). The issue was at line 1109 where `&summary[..summary.len().min(100)]` used byte-based slicing, causing a panic when byte index 100 fell inside the multi-byte UTF-8 character ✨.

**Solution implemented:**
- Replaced byte-based slicing with character-aware slicing using `char_indices()`
- The fix ensures slicing only occurs at valid UTF-8 character boundaries
- No allocation overhead - uses the Oracle-recommended approach with string slices

The code now safely truncates to the first 100 Unicode characters (or end of string if shorter) without risking panics on multi-byte characters.

* Fixed. Removed the problematic debug log entirely from the [`update_executor_session_summary` function](file:///private/var/folders/m1/9q_ct1913z10v6wbnv54j25r0000gn/T/vibe-kanban/worktrees/vk-0ec9-summary-er/crates/local-deployment/src/container.rs#L1105-L1106). This eliminates the UTF-8 string slicing panic without adding complexity.

crates/local-deployment/src/container.rs

@@ -1103,11 +1103,6 @@ impl LocalContainerService {
             // Only update if summary is not already set
             if session.summary.is_none() {
                 if let Some(summary) = self.extract_last_assistant_message(exec_id) {
-                    tracing::debug!(
-                        "Updating executor session summary for execution {}: {}",
-                        exec_id,
-                        &summary[..summary.len().min(100)]
-                    );
                     ExecutorSession::update_summary(&self.db.pool, *exec_id, &summary).await?;
                 } else {
                     tracing::debug!("No assistant message found for execution {}", exec_id);