Fix fm-spawn help parsing

[ThomShark]

Summary

• Handle fm-spawn.sh -h/--help before guard, backend, or positional parsing side effects.
• Add clean usage errors for missing spawn positionals instead of set -u crashes.
• Add regression coverage for fm-spawn.sh --help k- and isolate the spawn parser test from local dispatch config.
• Preserve no-mistakes fixes for environment-sensitive tests surfaced by the full validation run.

Validation

• bin/fm-spawn.sh --help k-
• tests/fm-spawn-batch.test.sh
• tests/fm-bootstrap.test.sh
• tests/fm-session-start.test.sh
• tests/fm-backend.test.sh
• shellcheck bin/*.sh bin/backends/*.sh tests/*.sh
• no-mistakes completed review, test, document, and lint successfully at 1953062d; upstream push failed with 403 for ThomShark, so this branch was pushed to the fork for PR creation.