fix: persist autostart PATH

[tuchg]

Summary

• Persist the PATH visible to lucarned autostart install into macOS LaunchAgent plist and Linux systemd unit files.
• Keep the service environment package-manager agnostic: no HOMEBREW_PATH and no unrelated environment snapshot.
• Document the daemon-startup PATH decision.

Tests

• cargo +nightly fmt --check
• env LUCARNE_AUTOSTART_EXPECT_PATH=1 cargo +nightly test -Zbuild-dir-new-layout -p lucarned-ctl service_path_env_reads_requested_current_process_path -- --quiet
• cargo +nightly check -Zbuild-dir-new-layout -p lucarned-ctl
• cargo +nightly test -Zbuild-dir-new-layout -p lucarned-ctl -- --quiet
• git diff --check

Closes #31