Lay the groundwork to support history in agent2 (#36483)

This pull request introduces title generation and history replaying. We
still need to wire up the rest of the history but this gets us very
close. I extracted a lot of this code from `agent2-history` because that
branch was starting to get long-lived and there were lots of changes
since we started.

Release Notes:

- N/A

crates/agent_servers/src/acp/v0.rs

@@ -1,4 +1,5 @@
 // Translates old acp agents into the new schema
+use action_log::ActionLog;
 use agent_client_protocol as acp;
 use agentic_coding_protocol::{self as acp_old, AgentRequest as _};
 use anyhow::{Context as _, Result, anyhow};
@@ -443,7 +444,8 @@ impl AgentConnection for AcpConnection {
             cx.update(|cx| {
                 let thread = cx.new(|cx| {
                     let session_id = acp::SessionId("acp-old-no-id".into());
-                    AcpThread::new(self.name, self.clone(), project, session_id, cx)
+                    let action_log = cx.new(|_| ActionLog::new(project.clone()));
+                    AcpThread::new(self.name, self.clone(), project, action_log, session_id)
                 });
                 current_thread.replace(thread.downgrade());
                 thread