fix(cli): propagate addon exit codes

[cedric013]

Summary

• make lean-ctx addon return status codes from addon subcommands instead of exiting inside addon-specific command modules
• propagate the addon status code from the top-level CLI dispatch
• keep invalid addon actions testable while preserving non-zero shell exit behavior

Closes #1275

Validation

• cargo fmt --check
• git grep std::process::exit(1) -- rust/src/cli/addon_cmd returns no matches
• cargo check --bin lean-ctx was started locally but exceeded the useful local wait window; CI will run the full check