AI observability guide: post_prompt_url and debug_webhook_url are undiscoverable

[briankwest]

Problem

Both of the AI agent's observability surfaces exist but are effectively undiscoverable from a debugging mindset:

• post_prompt_url is documented only as a configuration property on the ai method reference — nowhere near any discussion of logging, debugging, or observability. Its payload (conversation log, per-function SWAIG call log with arguments and results, call timeline including error entries, timings, summary) is exactly what a developer debugging an agent needs, and nothing connects the two.
• ai.params.debug_webhook_url — a real-time per-event stream (each SWAIG request/response exchange, voice errors, text normalization, post-prompt delivery) — is not documented near any debugging discussion either.

A developer evaluating the platform concluded "a developer debugging a SWML agent has essentially no server-side telemetry" — wrong, but a reasonable reading of the docs as organized.

Fix

1. An observability/debugging guide under the AI section leading with both features.
2. Reference documentation for the post_prompt payload fields (call_log, raw_call_log, call_timeline, swaig_log, global_data, post_prompt_data, timestamps, accounting totals).
3. Document debug_webhook_url / debug_webhook_level and the event envelope (call_info + per-event payload).
4. Cross-link from the ai reference pages.