daemon: fix UTF-8 panics on multi-byte character truncation

&str[..n] panics when n falls inside a multi-byte UTF-8 sequence.
This crashed the daemon when processing IRC messages containing
Hebrew/Yiddish characters (ehashman's messages hit byte 79-81).

Replace all byte-index truncation with chars().take(n).collect()
in tmux send_prompt preview, notification logging, and git context
truncation.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

src/bin/poc-daemon/context.rs

@@ -50,7 +50,7 @@ pub fn git_context() -> String {
     }
     let ctx = parts.join(" | ");
     if ctx.len() > 300 {
-        ctx[..300].to_string()
+        ctx.chars().take(300).collect()
     } else {
         ctx
     }