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/cli/mod.rs

@@ -1 +1,17 @@
+use clap::Parser;
 
+/// Interactive conventional commit tool for Jujutsu
+///
+/// Guides you through creating a properly formatted conventional commit message
+/// and applies it to the current change in your Jujutsu repository.
+#[derive(Debug, Parser)]
+#[command(
+    name = "jj-cz",
+    version,
+    about = "Interactive conventional commit tool for Jujutsu",
+    long_about = "Guides you through creating a properly formatted conventional \
+                  commit message and applies it to the current change in your \
+                  Jujutsu repository.\n\n\
+                  This tool requires an interactive terminal (TTY)."
+)]
+pub struct Cli;