`_append_node_dir_for_service()` had two bugs, both caught by
`test_system_unit_uses_target_user_home_not_calling_user`:
1. It crashed. `iter_hermes_node_dirs()` defaults to the *calling* user's
Hermes home, so under sudo it stats `/root/.hermes/node/bin` — which
raises `PermissionError` for a non-root caller rather than returning
False. An unreadable candidate dir means "skip this rung", not "kill
the generator", so the probe now swallows OSError.
2. Worse than the crash: had the stat succeeded, a `--system` unit
targeting alice would have baked *root's* managed Node into alice's
PATH. The generator now skips the managed-Node rung on the system
path and re-runs it after `_hermes_home_for_target_user()` resolves,
passing that home explicitly. Entries are prepended so the managed
Node still outranks the remapped shell-PATH entries, matching the
user-unit ordering.
The launchd generator is unaffected — it has no target-user remapping,
so the default home is already correct there.
0c3551849f