T24a: Agent, tool calls

- ToolCallCard: a step mark, the tool in mono, what it touched and how long
  it took; a press opens its arguments and result as code, an error in red
- ToolCallGroup folds several calls under one line
- ToolApprovalDialog over overlays::Dialog: Deny or Run, one answer; Escape,
  the scrim and the close deny
- PermissionPrompt: deny, always allow or allow once, in place
- chat::StepState gains Failed; chat::step_mark draws a step's mark for
  search steps and tool calls alike
- typography::format::took reads ms, seconds or minutes
- gallery: an Agent page with calls in each state, a group, the dialog and
  the prompt

src/agent/mod.rs

@@ -0,0 +1,7 @@
+mod permission;
+#[cfg(all(test, feature = "test-support"))]
+mod tests;
+mod tools;
+
+pub use permission::{Permission, PermissionPrompt, ToolApprovalDialog};
+pub use tools::{ToolCallCard, ToolCallGroup};