replace tail_messages with TailMessages iterator

TailMessages is a proper iterator that yields (role, text, timestamp)
newest-first. Owns the mmap internally. Caller decides when to stop.

resolve_conversation collects up to 200KB, then reverses to
chronological order. No compaction check needed — the byte budget
naturally limits how far back we scan.

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

poc-memory/src/agents/defs.rs

@@ -456,20 +456,30 @@ fn resolve_conversation() -> String {
     let Some(path) = transcript else { return String::new() };
     let path_str = path.to_string_lossy();
 
-    let messages = crate::transcript::tail_messages(&path_str, 200_000);
-    if messages.is_empty() { return String::new(); }
+    let Some(iter) = crate::transcript::TailMessages::open(&path_str) else {
+        return String::new();
+    };
 
     let cfg = crate::config::get();
-    let mut text = String::new();
-    for (role, content, ts) in &messages {
+    let mut fragments: Vec<String> = Vec::new();
+    let mut total_bytes = 0;
+    const MAX_BYTES: usize = 200_000;
+
+    for (role, content, ts) in iter {
+        if total_bytes >= MAX_BYTES { break; }
         let name = if role == "user" { &cfg.user_name } else { &cfg.assistant_name };
-        if !ts.is_empty() {
-            text.push_str(&format!("**{}** {}: {}\n\n", name, &ts[..ts.len().min(19)], content));
+        let formatted = if !ts.is_empty() {
+            format!("**{}** {}: {}", name, &ts[..ts.len().min(19)], content)
         } else {
-            text.push_str(&format!("**{}:** {}\n\n", name, content));
-        }
+            format!("**{}:** {}", name, content)
+        };
+        total_bytes += content.len();
+        fragments.push(formatted);
     }
-    text
+
+    // Reverse back to chronological order
+    fragments.reverse();
+    fragments.join("\n\n")
 }
 
 /// Get recently surfaced memory keys for the current session.