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/agent_ui/Cargo.toml

@@ -0,0 +1,107 @@
+[package]
+name = "agent_ui"
+version = "0.1.0"
+edition.workspace = true
+publish.workspace = true
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/agent_ui.rs"
+doctest = false
+
+[features]
+test-support = [
+    "gpui/test-support",
+    "language/test-support",
+]
+
+[dependencies]
+agent.workspace = true
+agent_settings.workspace = true
+anyhow.workspace = true
+assistant_context_editor.workspace = true
+assistant_slash_command.workspace = true
+assistant_slash_commands.workspace = true
+assistant_tool.workspace = true
+audio.workspace = true
+buffer_diff.workspace = true
+chrono.workspace = true
+client.workspace = true
+collections.workspace = true
+component.workspace = true
+context_server.workspace = true
+db.workspace = true
+editor.workspace = true
+extension.workspace = true
+extension_host.workspace = true
+feature_flags.workspace = true
+file_icons.workspace = true
+fs.workspace = true
+futures.workspace = true
+fuzzy.workspace = true
+gpui.workspace = true
+html_to_markdown.workspace = true
+indoc.workspace = true
+http_client.workspace = true
+indexed_docs.workspace = true
+inventory.workspace = true
+itertools.workspace = true
+jsonschema.workspace = true
+language.workspace = true
+language_model.workspace = true
+log.workspace = true
+lsp.workspace = true
+markdown.workspace = true
+menu.workspace = true
+multi_buffer.workspace = true
+notifications.workspace = true
+ordered-float.workspace = true
+parking_lot.workspace = true
+paths.workspace = true
+picker.workspace = true
+project.workspace = true
+prompt_store.workspace = true
+proto.workspace = true
+release_channel.workspace = true
+rope.workspace = true
+rules_library.workspace = true
+schemars.workspace = true
+search.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+serde_json_lenient.workspace = true
+settings.workspace = true
+smol.workspace = true
+streaming_diff.workspace = true
+telemetry.workspace = true
+telemetry_events.workspace = true
+terminal.workspace = true
+terminal_view.workspace = true
+text.workspace = true
+theme.workspace = true
+time.workspace = true
+time_format.workspace = true
+ui.workspace = true
+urlencoding.workspace = true
+util.workspace = true
+uuid.workspace = true
+watch.workspace = true
+workspace-hack.workspace = true
+workspace.workspace = true
+zed_actions.workspace = true
+zed_llm_client.workspace = true
+
+[dev-dependencies]
+assistant_tools.workspace = true
+buffer_diff = { workspace = true, features = ["test-support"] }
+editor = { workspace = true, features = ["test-support"] }
+gpui = { workspace = true, "features" = ["test-support"] }
+indoc.workspace = true
+language = { workspace = true, "features" = ["test-support"] }
+language_model = { workspace = true, "features" = ["test-support"] }
+pretty_assertions.workspace = true
+project = { workspace = true, features = ["test-support"] }
+rand.workspace = true