#39 wiki put: accept bare stdin (no literal - required)

SOURCE: swarm friction #5 (verified: printf x | f0rge wiki put docs/y.md -> usage error demanding the literal "-"; no --path flag either). FIX: single-arg form reads stdin — f0rge wiki put <path> with piped stdin stores it; - remains accepted for explicitness. wiki -h documents both forms. Probe: printf x | f0rge wiki put docs/y.md stores "x"; same with -. DONE: bare stdin works; docs truthful.