fix: resolve plugin paths from configured OpenRig home (#620)

AgentSpec plugin resource paths gain an openrig-home: form, resolved under the daemon's configured OpenRig home (OPENRIG_HOME, else the legacy RIGGED_HOME, else ~/.openrig) rather than the user's home directory, so a rig running under a non-default OpenRig home loads the plugin copy its daemon seeds. The suffix must be relative; an absolute suffix is refused, and the planning error reaches the instantiation result. Absolute, ~ and spec-relative paths are unchanged. The agent-spec reference documents the four path forms.