[Goodyslim]

Summary

Documents the webhook response classification already implemented in the listener: success, retryable, and permanent outcomes, the status-code matrix, and the bounded retry rule.

Problem

The code and tests classify 2xx, retryable 408/425/429/5xx plus timeout/network failures, and permanent client errors, but the rules were not documented (#643).

Solution

• Added listener/docs/webhook-response-classification.md with the outcome matrix, default-to-permanent rule, and retry bound.
• Linked it from the README's CI/quality documentation list.
• No runtime behavior changes: this documents the existing classifier and tests.

Validation

• Documentation-only change.
• Link target exists; no external links added.

Issue

Closes #643