feat(webhooks): expose safe local listener

[caiopizzol]

What changed

• exposes amp listen;
• logs safe delivery metadata by default;
• adds --show-payload for full webhook bodies;
• makes forwarding optional; and
• keeps public routing and destination changes outside the listener.

Why

Developers and agents need to verify real Read and Subscribe deliveries locally without printing provider field values by default.

Tested

• Full Go test suite, vet, and golangci-lint 2.9.0
• Real HubSpot contact-created Subscribe delivery
• One item received without contact fields in output
• No-forwarding response verified as HTTP 204
• Disposable contact removed and destination restored