docs(examples): webhook wiring recipe — handler-authoring section + expanded hello_seller + new with_webhooks example

[bokelley]

Closes #546

Summary

Three non-breaking documentation and examples changes that together close the silent-no-op trap for first-time DecisioningPlatform adopters:

• docs/handler-authoring.md: adds a new ## Webhooks section covering the three sender constructors (comparison table), the sender-vs-supervisor distinction, a wiring snippet, and cross-links to the migration guide and the new example. The section makes docs/handler-authoring.md#webhooks a live anchor (previously the issue proposed linking there but the section didn't exist).
• examples/hello_seller.py: expands the three-line "wire webhook_sender= in production" note into a full three-constructor menu with import lines, a JWK field reminder, and a link to docs/handler-authoring.md#webhooks.
• examples/hello_seller_with_webhooks.py (new): ~57-LOC runnable wiring example using WebhookSender.from_bearer_token + InMemoryWebhookDeliverySupervisor, the first copy-paste-ready path from bare handler to production-grade webhooks.

Note: the boot-time fail-fast (validate_webhook_sender_for_platform) was already shipping as a hard AdcpError raise — this PR closes the documentation gap that let adopters reach that error without knowing how to resolve it.

What was tested

• pytest (3866 passed, 32 skipped, 1 pre-existing network test failure on signing/test_ip_pinned_transport.py::test_real_tls_handshake_still_validates_hostname — unrelated to this PR, pre-exists on main)
• mypy src/adcp/ — 878 errors, identical to main (no source files changed)
• ruff check src/ — all checks passed

Pre-PR review

Two expert passes were run (blockers fixed before final sign-off):

• code-reviewer: approved — prior blocker (silent dev-fixture-token fallback) resolved with warnings.warn; 1 nit noted in PR body
• dx-expert: approved — prior blocker (from_jwk kwargs mismatch) resolved; kid/alg/adcp_use now correctly documented as JWK dict fields

Nits surfaced (not fixed):

• examples/README.md does not list hello_seller_with_webhooks.py — consistent with how other hello_seller_*.py variants are handled (none appear to be individually listed); can be addressed in a follow-on README update pass.

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_01Ay2SJvRpBg8EP9nG9DgQMy

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