Find managed-install plugins in agent-help delegation

main prepends the managed plugin bin dir only around the pre-Cobra dispatcher and restores PATH before Cobra runs, so agent-help's resolvePlugin never saw entire plugin install plugins and reported them as unknown commands. Prepend it again for the lookup and the run (the child inherits it, as a dispatched plugin does), then restore.

Also correct the contract: agent-help's own post-run (command telemetry, stderr version notice) still fires for a delegated lookup, as for any lookup; only plugin-invocation telemetry is skipped. Command telemetry records no positional args, so the plugin's name is never sent. Document that agent-help takes a command path, so other flags are rejected before the plugin is consulted, as they are for built-ins.