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

src/agent/mod.rs

@@ -1,9 +1,11 @@
+mod changes;
 mod permission;
 mod progress;
 #[cfg(all(test, feature = "test-support"))]
 mod tests;
 mod tools;
 
+pub use changes::{ChangeState, FileChange, FileChangeCard, MultiFileDiffReview};
 pub use permission::{Permission, PermissionPrompt, ToolApprovalDialog};
 pub use progress::{AgentPlan, AgentProgress, AgentState, AgentStatus, AgentStep, AgentStepList};
 pub use tools::{ToolCallCard, ToolCallGroup};