Add /clear command

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server/src/runner.rs

@@ -1723,10 +1723,43 @@ async fn compact_inner(app: Arc<App>, session_id: &str, cancel: CancellationToke
     Ok(())
 }
 
+/// Text of the marker `/clear` puts into the history (the chat view recognises it).
+pub const CLEARED_MARKER: &str = "[Context cleared by the user. Earlier messages are no longer part of this conversation – start fresh and do not refer to them.]";
+
+/// `/clear`: the model forgets the conversation so far (the chat keeps showing it). Like a
+/// compaction without a summary: history before the marker is not sent any more.
+pub fn clear_context(app: &App, session_id: &str) -> Result<()> {
+    save_user_parts(app, session_id, vec![Part::Summary { text: CLEARED_MARKER.into(), auto: false, tokens: None }])?;
+    app.update_session(session_id, |s| {
+        s.last_usage = None;
+        s.todos = Value::Null;
+    })?;
+    app.with_runtime(session_id, |r| r.read_files.clear());
+    // Claude Code continues its own conversation: start a new one.
+    if let Some(owner) = app.session(session_id).ok().and_then(|s| s.owner_id) {
+        let _ = app.store.set_user_setting(&owner, &format!("claude_sid:{session_id}"), &Option::<String>::None);
+    }
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn cleared_marker_starts_a_new_history() {
+        let mk = |role, parts| Message::new(role, parts);
+        let all = vec![
+            mk(Role::User, vec![Part::Text { text: "old question".into() }]),
+            mk(Role::Assistant, vec![Part::Text { text: "old answer".into() }]),
+            mk(Role::User, vec![Part::Summary { text: CLEARED_MARKER.into(), auto: false, tokens: None }]),
+            mk(Role::User, vec![Part::Text { text: "new question".into() }]),
+        ];
+        let active = active_history(all);
+        assert_eq!(active.len(), 2);
+        assert!(!active.iter().any(|m| message_text(m).contains("old")));
+    }
+
     #[test]
     fn fit_keeps_head_and_tail_within_budget() {
         let text = format!("{}{}{}", "A".repeat(5000), "M".repeat(5000), "Z".repeat(5000));