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-planner/src/history.rs

@@ -137,8 +137,9 @@ pub fn write_completed_requirements(
 pub fn write_git_commit(plan_dir: &Path, message: &str) -> Result<()> {
     let timestamp = format_timestamp();
     // Truncate message if too long for a single line
-    let truncated_message = if message.len() > 72 {
-        format!("{}...", &message[..69])
+    let truncated_message = if message.chars().count() > 72 {
+        let chars: String = message.chars().take(69).collect();
+        format!("{}...", chars)
     } else {
         message.to_string()
     };