T24b: Agent, progress

- AgentStepList: an agent's steps down a data_display::Timeline rail,
  marked by chat::step_mark, with detail and time
- AgentPlan: items waiting, at work or done, with a count; done items go
  quiet and struck through
- chat::step_mark reads motion::changes: a check or cross that arrives pops
  in, still on first paint
- AgentStatus: a pill for planning, working, waiting, done or failed
- AgentProgress: the step at work, done of total, the time so far and a
  bar split by step; progress past its total fails loud
- SubAgentTree is lists::Tree with each agent's state as its note
- gallery: a plan the button advances, steps, statuses and a tree

src/agent/mod.rs

@@ -1,7 +1,9 @@
 mod permission;
+mod progress;
 #[cfg(all(test, feature = "test-support"))]
 mod tests;
 mod tools;
 
 pub use permission::{Permission, PermissionPrompt, ToolApprovalDialog};
+pub use progress::{AgentPlan, AgentProgress, AgentState, AgentStatus, AgentStep, AgentStepList};
 pub use tools::{ToolCallCard, ToolCallGroup};