Extract an agent_ui crate from agent (#33284)

This PR moves the UI-dependent logic in the `agent` crate into its own
crate, `agent_ui`. The remaining `agent` crate no longer depends on
`editor`, `picker`, `ui`, `workspace`, etc.

This has compile time benefits, but the main motivation is to isolate
our core agentic logic, so that we can make agents more
pluggable/configurable.

Release Notes:

- N/A

Cargo.toml

@@ -2,6 +2,7 @@
 resolver = "2"
 members = [
     "crates/activity_indicator",
+    "crates/agent_ui",
     "crates/agent",
     "crates/agent_settings",
     "crates/anthropic",
@@ -214,6 +215,7 @@ edition = "2024"
 
 activity_indicator = { path = "crates/activity_indicator" }
 agent = { path = "crates/agent" }
+agent_ui = { path = "crates/agent_ui" }
 agent_settings = { path = "crates/agent_settings" }
 ai = { path = "crates/ai" }
 anthropic = { path = "crates/anthropic" }