T24c: Agent, changes

- FileChangeCard: a proposed change to one file, its path and counts,
  Reject and Accept in a slot that keeps presses off the header; opened,
  the diff through a headless git::DiffViewer with its folds opening
- MultiFileDiffReview: every file at once, the total counts, how many are
  reviewed, Reject all and Accept all; verdicts as collab::Decision
- git::DiffViewer gains headless, leaving the file's header to its owner
- CommandExecutionCard is terminal::CommandBlock
- gallery: three proposed files and a command's block

tasks/ch21-30.md

@@ -179,9 +179,9 @@
 
 ## T24c Agent · Changes
 
-- [ ] FileChangeCard
-- [ ] MultiFileDiff Review
-- [ ] CommandExecutionCard
+- [x] FileChangeCard
+- [x] MultiFileDiff Review → `agent::MultiFileDiffReview`, verdicts as `collab::Decision`
+- [x] CommandExecutionCard → `terminal::CommandBlock`
 
 ## T24d Agent · Previews