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/src/slash_command_settings.rs

@@ -1,49 +0,0 @@
-use anyhow::Result;
-use gpui::App;
-use schemars::JsonSchema;
-use serde::{Deserialize, Serialize};
-use settings::{Settings, SettingsSources};
-
-/// Settings for slash commands.
-#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema)]
-pub struct SlashCommandSettings {
-    /// Settings for the `/docs` slash command.
-    #[serde(default)]
-    pub docs: DocsCommandSettings,
-    /// Settings for the `/cargo-workspace` slash command.
-    #[serde(default)]
-    pub cargo_workspace: CargoWorkspaceCommandSettings,
-}
-
-/// Settings for the `/docs` slash command.
-#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema)]
-pub struct DocsCommandSettings {
-    /// Whether `/docs` is enabled.
-    #[serde(default)]
-    pub enabled: bool,
-}
-
-/// Settings for the `/cargo-workspace` slash command.
-#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema)]
-pub struct CargoWorkspaceCommandSettings {
-    /// Whether `/cargo-workspace` is enabled.
-    #[serde(default)]
-    pub enabled: bool,
-}
-
-impl Settings for SlashCommandSettings {
-    const KEY: Option<&'static str> = Some("slash_commands");
-
-    type FileContent = Self;
-
-    fn load(sources: SettingsSources<Self::FileContent>, _cx: &mut App) -> Result<Self> {
-        SettingsSources::<Self::FileContent>::json_merge_with(
-            [sources.default]
-                .into_iter()
-                .chain(sources.user)
-                .chain(sources.server),
-        )
-    }
-
-    fn import_from_vscode(_vscode: &settings::VsCodeSettings, _current: &mut Self::FileContent) {}
-}