Fix UTF-8 panics and inconsistent retry logic

- Fix 7 UTF-8 byte slicing panics that crash on multi-byte characters:
  - acd.rs: extract_topic_from_text() [..50] slice
  - streaming.rs: log_stream_error() [..500] slice
  - tools/acd.rs: rehydrate message truncation [..2000] slice
  - history.rs: git commit message truncation [..69] slice
  - planner.rs: commit summary/description truncation [..69] slices
  - llm.rs: requirements summary line truncation [..117] slice

- All now use chars().count() and chars().take(N).collect() for
  UTF-8 safe truncation

- Fix inconsistent retry logic in task_execution.rs:
  - Previously only retried on Timeout errors
  - Now retries on ALL recoverable errors (rate limits, network,
    server errors, model busy, token limits, context length)
  - Added error-specific base delays (rate limit: 5s, server: 2s, etc.)
  - Added exponential backoff with ±20% jitter
  - Consistent with autonomous mode retry behavior

crates/g3-core/src/streaming.rs

@@ -200,8 +200,9 @@ pub fn log_stream_error(
         .rev()
         .find(|m| matches!(m.role, MessageRole::User))
     {
-        let truncated = if last_user_msg.content.len() > 500 {
-            format!("{}... (truncated)", &last_user_msg.content[..500])
+        let truncated = if last_user_msg.content.chars().count() > 500 {
+            let chars: String = last_user_msg.content.chars().take(500).collect();
+            format!("{}... (truncated)", chars)
         } else {
             last_user_msg.content.clone()
         };