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/Cargo.toml

@@ -18,6 +18,7 @@ doctest = false
 
 [dependencies]
 acp_thread.workspace = true
+action_log.workspace = true
 agent-client-protocol.workspace = true
 agent_settings.workspace = true
 agentic-coding-protocol.workspace = true