fix: refresh service help backend text (#383)

## Summary

Refresh `all-smi service --help` so it matches the shipped backend set from PR #321 and keeps the `--user` scope wording accurate.

## What changed

- Update `src/cli.rs` service help text to name Linux systemd, macOS launchd, and Windows SCM as supported backends.
- Clarify in the same help text that `--user` applies to Linux and macOS, not Windows.
- Add a focused `src/cli.rs` regression test that fails if the help text again claims launchd is tracked separately or omits macOS support.

## Homebrew verification context

- On August 23, 2026, the tap `service do` block and Homebrew-installed binary refusal path from #310 were manually verified outside this repo.
- The pre-service-block 0.25.0 Homebrew receipt upgrade path was explicitly left out of scope because the next release will carry the new metadata; this PR does not add a tap revision or compatibility workaround for existing receipts.

## Test plan

- [x] `cargo test --lib cli::tests`
- [x] `cargo fmt --check`
- [x] `cargo run --quiet --bin all-smi -- service --help`

Closes #355