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

tasks/ch21-30.md

@@ -164,10 +164,10 @@
 
 ## T24a Agent · Tool calls — `src/agent`
 
-- [ ] ToolCallCard
-- [ ] ToolCallGroup
-- [ ] ToolApprovalDialog
-- [ ] PermissionPrompt
+- [x] ToolCallCard
+- [x] ToolCallGroup
+- [x] ToolApprovalDialog
+- [x] PermissionPrompt
 
 ## T24b Agent · Progress