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/lib.rs

@@ -3,3 +3,22 @@ mod commit;
 mod error;
 mod jj;
 mod prompts;
+
+pub use crate::{
+    commit::types::{
+        CommitMessageError, CommitType, ConventionalCommit, Description, DescriptionError, Scope,
+        ScopeError,
+    },
+    error::Error,
+    jj::{JjExecutor, lib_executor::JjLib},
+    prompts::{CommitWorkflow, Prompter},
+};
+
+/// Test utilities: mock implementations for `JjExecutor` and `MockPrompts`.
+///
+/// Enable with `--features test-utils` (e.g. `cargo test --features test-utils`).
+#[cfg(feature = "test-utils")]
+pub use crate::{
+    jj::mock::MockJjExecutor,
+    prompts::mock::MockPrompts,
+};