Add AI sidecar guide to the SWML guides

[Manny-r31]

Summary

• Adds Coach live calls with an AI sidecar (/docs/swml/guides/ai-sidecar), a basics guide for the ai_sidecar SWML method. Its structure and phrasing follow the call queues guide in docs(voice): add call queues guide #780.
• Running example: a generic support call. The sidecar sends the agent one short tip after each customer turn. It uses no tools, to keep the sample minimal; tools are linked to the AI sidecar SWAIG reference.
• Shared sections:
  • The SWML and REST comparison table, prepare, and how the AI sidecar works.
  • Run the sidecar server: a short Python or TypeScript SWMLService that prints sidecar callbacks at /events.
• Coach a call via SWML: the document goes in a hosted SWML Script, whose YAML and JSON the reader pastes into the Dashboard. The reader assigns it to a number and calls between two phones.
• Coach a call in progress via REST: attach a sidecar by call ID, then stop it while the call continues.
• Left to the reference pages and Next steps cards: REST ask and poke, tools and tool actions, and options.
• Nav and index: the page lives in the folder-driven Recipes section of the SWML Guides tab, so swml.yml is unchanged. The fern/llms.txt AI sidecar entry now points at the guide.

Notes for reviewers

• Hosted script, not served SWML: the document is static, so a hosted SWML Script delivers it. The server only receives callbacks, which avoids an External URL with credentials embedded in it.
• SDK gap: neither SDK's REST client wraps the calling.ai_sidecar* commands, so the REST section uses the existing EndpointRequestSnippet examples. Per author review, the page doesn't call out the gap.
• TypeScript SDK quirk the sample works around: registerRoutingCallback redirects unless the callback returns exactly null, so the sample returns null, never undefined.
• Facts trace to the platform source (mod_openai sidecar implementation, mod_infrastructure schema), the TypeSpec, and the existing reference pages. Internal detail is kept out: engine, transport, Relay topic names, and billing SKUs.
  • ai_sidecar answers the call itself, so the document has no answer.
  • After calling.ai_sidecar.stop, the call can take a new sidecar.
• The existing ai_sidecar reference pages disagree with the source in places. The guide avoids these claims, and this PR doesn't fix them; that's a follow-up:
  • They say a sidecar and live_transcribe can't run on one call. The source allows it.
  • They list prompt variables (${global_data.x}, ${local_date}, ${session_uuid}) that the source mostly doesn't support.
  • They say params.transcribe_prompt biases speech recognition. It never reaches the speech engine.
  • permissions.swaig_set_global_data isn't enforced.
  • The settings, back_to_back_functions, extensive_data, and set_meta_data actions do nothing in a sidecar.
  • They say final is always the last callback. After a transfer or hangup action, it arrives before stop.
  • The reference index exposes internal detail: the Relay topic name, curl_code, and the speech engine vendor.
• Code block titles keep the template's <Language> — <client> format.

Verification

• yarn fern-md-check: passes (all MDX files valid).
• yarn fern-check: only the Fern-auth-gated "missing redirects" check fails locally because this machine isn't logged in to Fern. The one warning is the existing accent-color contrast warning.
• Samples were reviewed statically against the pinned SDK source; none were run. The author is testing them against a live call.
  • Python signalwire-sdk==3.4.1: SWMLService basic auth and register_routing_callback. The callback runs on a non-empty POST, after the auth check.
  • TypeScript @signalwire/sdk@2.0.5: SWMLService basic auth and registerRoutingCallback.
• Internal links are checked against slug: frontmatter. Anchors are checked by hand: #callback-types, #answer-a-call-via-swml, #prepare-for-your-first-call, and the in-page anchors.

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