fix(agent): restore compile-time signal_routes and preserve callback compatibility

[mikehostetler]

Summary

• fix real bug where signal_routes: passed to use Jido.Agent was silently ignored
• add first-class signal_routes option to agent config schema with default []
• preserve advanced callback usage and add compatibility for legacy signal_routes/0
  • default signal_routes/0 now returns compile-time routes
  • default signal_routes/1 delegates to signal_routes/0
  • both arities are optional callbacks and overridable
• keep runtime routing contract stable (SignalRouter still calls signal_routes/1 with %{agent_module: module})
• align callback docs to actual runtime context shape
• sync docs/examples to compile-time signal_routes: style for static routing
• add regression tests for:
  • compile-time routes via use Jido.Agent, signal_routes: [...]
  • /1 override extending compile-time routes via super(ctx)
  • legacy /0 callback compatibility through delegated /1
  • signal router builds routes correctly for both compile-time and legacy callback agents

Validation

• mix compile
• mix test
• mix quality
• mix test test/jido/agent/agent_test.exs
• mix test test/jido/agent_server/signal_router_test.exs
• mix test --include example test/examples/signals/context_aware_routing_test.exs