feat: add interactive conventional commit workflow with jj-lib backend

Replace CLI executor with jj-lib integration, implement full interactive
commit workflow via prompts, and add mock infrastructure for testing.
Add CLI integration tests and error handling tests.

src/prompts/mod.rs

@@ -1 +1,7 @@
+#[cfg(any(test, feature = "test-utils"))]
+pub mod mock;
+pub mod prompter;
+pub mod workflow;
 
+pub use prompter::Prompter;
+pub use workflow::CommitWorkflow;