Fix addon CLI exit code propagation

[cedric013]

Problem

lean-ctx addon command handlers should not terminate the process from inside addon-specific logic. The addon command already returns an integer status code, but the top-level CLI dispatch path must propagate that code so failures from addon subcommands reach the shell.

Expected

• addon subcommands remain testable and return status codes instead of calling process::exit internally
• top-level lean-ctx addon ... exits non-zero when addon command handling returns non-zero
• invalid addon actions report the error and exit with code 1

Validation

• lean-ctx addon unknown-action should return exit code 1 and print addon help
• no process::exit calls should remain in rust/src/cli/addon_cmd.rs