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
     }

src/bin/poc-daemon/notify.rs

@@ -169,7 +169,7 @@ impl NotifyState {
 
         info!(
             "notification: type={ntype} urgency={urgency} threshold={threshold} msg={}",
-            &message[..message.len().min(80)]
+            message.chars().take(80).collect::<String>()
         );
 
         self.pending.push(Notification {

src/bin/poc-daemon/tmux.rs

@@ -32,7 +32,8 @@ pub fn find_claude_pane() -> Option<String> {
 ///
 /// Sequence: Escape q C-c C-u (clear input), wait, type message, Enter.
 pub fn send_prompt(pane: &str, msg: &str) -> bool {
-    info!("SEND [{pane}]: {}...", &msg[..msg.len().min(100)]);
+    let preview: String = msg.chars().take(100).collect();
+    info!("SEND [{pane}]: {preview}...");
 
     let send = |keys: &[&str]| {
         Command::new("tmux")