Follow-ups on salvaged #81419:
- Honor the plugins.enabled allow-list / plugins.disabled deny-list (same
opt-in contract as the general PluginManager) — installed != loaded.
- Skip callables that require arguments: general plugins share the
hermes_agent.plugins group with register(ctx) targets; invoking them
zero-arg would TypeError-spam every startup.
- Fix test docstring (entry points are discovered FIRST, lowest precedence)
and docs mechanism wording; document the config gate.
- New tests: opt-in gate, deny-list, register(ctx) never invoked.
E2E-verified with a real pip-built package against a temp HERMES_HOME.
