Load journal from memory graph, not flat file

Replace flat-file journal parser with direct store query for
EpisodicSession nodes. Filter journal entries to only those older
than the oldest conversation message (plus one overlap entry to
avoid gaps). Falls back to 20 recent entries when no conversation
exists yet.

Fixes: poc-agent context window showing 0 journal entries.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/log.rs

@@ -125,4 +125,26 @@ impl ConversationLog {
     pub fn path(&self) -> &Path {
         &self.path
     }
+
+    /// Get the timestamp of the oldest message in the log.
+    pub fn oldest_timestamp(&self) -> Option<chrono::DateTime<chrono::Utc>> {
+        let file = File::open(&self.path).ok()?;
+        let reader = BufReader::new(file);
+        for line in reader.lines().flatten() {
+            let line = line.trim().to_string();
+            if line.is_empty() { continue; }
+            if let Ok(msg) = serde_json::from_str::<Message>(&line) {
+                if let Some(ts) = &msg.timestamp {
+                    if let Ok(dt) = chrono::DateTime::parse_from_rfc3339(ts) {
+                        return Some(dt.to_utc());
+                    }
+                    // Try other formats
+                    if let Ok(dt) = chrono::NaiveDateTime::parse_from_str(ts, "%Y-%m-%dT%H:%M:%S") {
+                        return Some(dt.and_utc());
+                    }
+                }
+            }
+        }
+        None
+    }
 }