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

crates/zed/Cargo.toml

@@ -21,6 +21,7 @@ path = "src/main.rs"
 [dependencies]
 activity_indicator.workspace = true
 agent.workspace = true
+agent_ui.workspace = true
 agent_settings.workspace = true
 anyhow.workspace = true
 askpass.workspace = true