Mirror the events layering: the framework/SPI stays in internal/agent, the
concrete business providers move to a top-level agent/ package (agent/example/),
and agent/register.go blank-imports each so their init() self-registration runs.
cmd/build.go blank-imports the top-level agent package (alongside events); the
command layer (cmd/agent) no longer wires providers directly.
A test-only blank import keeps the example scheme registered for cmd/agent
tests, which exercise example:echo / example:reporter offline.
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