docs(README): note dual token-shape support + LOG_LEVEL tuning

[mabry1985]

Summary

Two operator-visible tweaks in the README's streaming + push notifications section, following #64 + #62:

• Token-parsing accepts both spec-legal shapes (top-level `token` used by @a2a-js/sdk / Workstacean, and structured `authentication.credentials` for RFC-8821-style callers).
• `LOG_LEVEL=INFO` (the default) surfaces every push-config registration + webhook delivery attempt with task id + state + response status.

Closes

Test plan

• Renders cleanly

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