Delegate `entire agent-help <plugin>` to the plugin's own agent-help

When <name> is not a built-in and entire-<name> resolves, agent-help runs entire-<name> agent-help [args...] [--json] instead of reporting an unknown command, so plugins document themselves through the same drill-in agents use for built-ins.

Resolution and launch reuse the dispatcher's resolvePlugin and runPlugin rather than a bare LookPath, so built-ins still win, agent- names stay reserved, and the child gets the filtered plugin environment. A missing on-demand plugin is not offered for install (a help lookup must not start a download), and no invocation telemetry or version notice fires. The MCP agent_help tool does not delegate: the plugin writes to stdout, which is the JSON-RPC stream under entire mcp.