feat: Move debug log truncation from agent loop to REPL channel (#65)

* feat: Move debug log truncation from agent loop to REPL channel

Full tool output now flows through StatusUpdate so the web gateway
gets untruncated content. The REPL channel truncates at display time
(200 chars for tool results, thinking, and status messages).

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* feat: truncating fmt layer for terminal, full logs for web gateway

Instead of truncating debug output at each LLM call site (fragile),
use a custom MakeWriter on the fmt layer that caps each tracing event
at 500 bytes before flushing to stderr. The web gateway WebLogLayer
still receives full untruncated content for /api/logs/events SSE.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix: UTF-8 safe truncation in truncate_for_preview, remove double truncation

- Use char_indices() instead of byte-based slicing to find the cut
  point, preventing panics on multi-byte characters (emoji, CJK, etc.)
- Remove redundant truncation in REPL channel (agent loop already
  truncates ToolResult previews to 200 chars)
- Add 9 unit tests covering edge cases: empty, exact length, multi-byte
  UTF-8 (emoji, CJK), mixed scripts, newline collapsing, whitespace

Addresses PR #65 review comments.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/agent/agent_loop.rs

@@ -28,7 +28,7 @@ use crate::tools::ToolRegistry;
 use crate::workspace::Workspace;
 
 /// Collapse a tool output string into a single-line preview for display.
-fn truncate_for_preview(output: &str, max_chars: usize) -> String {
+pub(crate) fn truncate_for_preview(output: &str, max_chars: usize) -> String {
     let collapsed: String = output
         .chars()
         .take(max_chars + 50)
@@ -37,8 +37,14 @@ fn truncate_for_preview(output: &str, max_chars: usize) -> String {
         .split_whitespace()
         .collect::<Vec<_>>()
         .join(" ");
-    if collapsed.len() > max_chars {
-        format!("{}...", &collapsed[..max_chars])
+    // char_indices gives us byte offsets at char boundaries, so the slice is always valid UTF-8.
+    if collapsed.chars().count() > max_chars {
+        let byte_offset = collapsed
+            .char_indices()
+            .nth(max_chars)
+            .map(|(i, _)| i)
+            .unwrap_or(collapsed.len());
+        format!("{}...", &collapsed[..byte_offset])
     } else {
         collapsed
     }
@@ -2627,4 +2633,70 @@ mod tests {
 
         assert!(detect_auth_awaiting("tool_activate", &result).is_none());
     }
+
+    // --- truncate_for_preview tests ---
+
+    use super::truncate_for_preview;
+
+    #[test]
+    fn test_truncate_short_input() {
+        assert_eq!(truncate_for_preview("hello", 10), "hello");
+    }
+
+    #[test]
+    fn test_truncate_empty_input() {
+        assert_eq!(truncate_for_preview("", 10), "");
+    }
+
+    #[test]
+    fn test_truncate_exact_length() {
+        assert_eq!(truncate_for_preview("hello", 5), "hello");
+    }
+
+    #[test]
+    fn test_truncate_over_limit() {
+        let result = truncate_for_preview("hello world, this is long", 10);
+        assert!(result.ends_with("..."));
+        // "hello worl" = 10 chars + "..."
+        assert_eq!(result, "hello worl...");
+    }
+
+    #[test]
+    fn test_truncate_collapses_newlines() {
+        let result = truncate_for_preview("line1\nline2\nline3", 100);
+        assert!(!result.contains('\n'));
+        assert_eq!(result, "line1 line2 line3");
+    }
+
+    #[test]
+    fn test_truncate_collapses_whitespace() {
+        let result = truncate_for_preview("hello   world", 100);
+        assert_eq!(result, "hello world");
+    }
+
+    #[test]
+    fn test_truncate_multibyte_utf8() {
+        // Each emoji is 4 bytes. Truncating at char boundary must not panic.
+        let input = "😀😁😂🤣😃😄😅😆😉😊";
+        let result = truncate_for_preview(input, 5);
+        assert!(result.ends_with("..."));
+        // First 5 chars = 5 emoji
+        assert_eq!(result, "😀😁😂🤣😃...");
+    }
+
+    #[test]
+    fn test_truncate_cjk_characters() {
+        // CJK chars are 3 bytes each in UTF-8.
+        let input = "你好世界测试数据很长的字符串";
+        let result = truncate_for_preview(input, 4);
+        assert_eq!(result, "你好世界...");
+    }
+
+    #[test]
+    fn test_truncate_mixed_multibyte_and_ascii() {
+        let input = "hello 世界 foo";
+        let result = truncate_for_preview(input, 8);
+        // 'h','e','l','l','o',' ','世','界' = 8 chars
+        assert_eq!(result, "hello 世界...");
+    }
 }

src/agent/mod.rs

@@ -26,6 +26,7 @@ pub mod task;
 pub mod undo;
 pub mod worker;
 
+pub(crate) use agent_loop::truncate_for_preview;
 pub use agent_loop::{Agent, AgentDeps};
 pub use compaction::{CompactionResult, ContextCompactor};
 pub use context_monitor::{CompactionStrategy, ContextBreakdown, ContextMonitor};