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.

Cargo.toml

@@ -15,18 +15,25 @@ path = "src/lib.rs"
 path = "src/main.rs"
 name = "jj-cz"
 
+[features]
+## Exposes MockJjExecutor and MockPrompts for use in integration tests.
+## Enable with: cargo test --features test-utils
+test-utils = []
+
 [dependencies]
-assert_cmd = "2.1.2"
-assert_fs = "1.1.3"
 async-trait = "0.1.89"
 clap = { version = "4.5.57", features = ["derive"] }
 git-conventional = "0.12.9"
 inquire = "0.9.2"
-jj-lib = { version = "0.38.0", features = ["testing"] }
+jj-lib = "0.39.0"
 lazy-regex = { version = "3.5.1", features = ["lite"] }
-predicates = "3.1.3"
 thiserror = "2.0.18"
 tokio = { version = "1.49.0", features = ["macros", "rt-multi-thread"] }
 
+[dev-dependencies]
+assert_cmd = "2.1.2"
+assert_fs = "1.1.3"
+predicates = "3.1.3"
+
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(tarpaulin_include)'] }