agent: fix — add missing transcribe-rs fields (leading/trailing silence)

Upstream transcribe-rs 0.3.10 added required fields to TranscribeOptions.
Set both to None (use engine defaults).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/transcription/src/local_engine.rs

@@ -78,6 +78,8 @@ impl LocalEngine {
         let opts = TranscribeOptions {
             language: options.language.clone(),
             translate: false,
+            leading_silence_ms: None,
+            trailing_silence_ms: None,
         };
 
         let start = Instant::now();