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

tasks/ch21-30.md

@@ -171,11 +171,11 @@
 
 ## T24b Agent · Progress
 
-- [ ] AgentStepList / TaskTimeline
-- [ ] AgentPlan / TodoList
-- [ ] AgentStatus
-- [ ] AgentProgress
-- [ ] SubAgentTree
+- [x] AgentStepList / TaskTimeline → `agent::AgentStepList`, over `data_display::Timeline`
+- [x] AgentPlan / TodoList → `agent::AgentPlan`, marks that pop in as items finish
+- [x] AgentStatus
+- [x] AgentProgress
+- [x] SubAgentTree → `lists::Tree`, a node per agent with its state as note and tone
 
 ## T24c Agent · Changes