refactor(hm-plugin-cloud): Replace raw i32 process exit codes with a typed ExitCode enum

[markovejnovic]

The smell

dispatch and dispatch_command in crates/hm-plugin-cloud/src/cli.rs return Result<i32> and write process exit codes as bare integer literals: Ok(0) (success), Ok(2) (arg-parse failure), and Ok(1) (runtime error). The meaning of each number was implicit and the same magic values were duplicated across both functions, so a reader had to know the convention and a typo could silently return an undefined status.

The change

Introduces a small private enum ExitCode { Success, RuntimeError, UsageError } with a single From<ExitCode> for i32 impl mapping the variants to 0 / 1 / 2. The dispatch arms now produce ExitCode::Success.into(), ExitCode::UsageError.into(), and ExitCode::RuntimeError.into() instead of bare integers. The integer mapping is unchanged, so behavior is preserved.

Type-safety pattern

This applies fd's "typed exit code enum with From<ExitCode> for i32" pattern (see the fd repository, sharkdp/fd, src/exit_codes.rs): the magic 0/1/2 appears exactly once, each code is self-documenting at every call site, and the exhaustive match in the From impl prevents accidentally introducing an undefined status.

Validation

Only cargo check -p hm-plugin-cloud was run locally (passes). Full CI runs on this PR. Draft because it still needs review.