feat(whatsapp): add WhatsApp Business Cloud API adapter

Add an official, production-grade WhatsApp integration via Meta's
Business Cloud API as a complement to the existing Baileys bridge.
No bridge subprocess, no QR codes, no account-ban risk — at the cost
of a Meta Business account and a public HTTPS webhook URL.

Setup is fully wizard-driven: 'hermes whatsapp-cloud' walks through
every credential with paste-time validation (catches the #1 trap of
pasting a phone number into the Phone Number ID field), generates a
verify token, and ends with copy-paste instructions for the
cloudflared / Meta-dashboard / Business Manager pieces that can't be
automated. The wizard also points users at Meta's Business Manager
for setting the bot's display name and profile picture.

Feature set:

- Inbound: text, images (with native-vision routing), voice notes
  (STT), documents (small text inlined, larger cached), reply context.
- Outbound: text with WhatsApp-flavored markdown conversion, images,
  videos, documents, opus voice notes via ffmpeg with MP3 fallback.
- Native interactive buttons for clarify, dangerous-command approval,
  and slash-command confirmation flows — matches the Telegram /
  Discord UX, graceful degrades to plain text.
- Read receipts (blue double-checkmarks) and typing indicator,
  using Meta's combined endpoint so they fire in a single API call.
- Webhook security: X-Hub-Signature-256 HMAC verification (raw body,
  constant-time), wamid deduplication, group-shaped-message refusal
  (groups deferred to v2 — Baileys still covers them).
- Full integration with the gateway's session, cron, display-tier,
  prompt-hint, and auth-allowlist systems. Cloud and Baileys can run
  side-by-side against different phone numbers.

Also wires STT (speech-to-text) through Nous's managed audio gateway
for Nous subscribers — previously the default stt.provider=local
required a separate faster-whisper install. New subscribers now get
voice-note transcription out of the box.

Docs: 418-line user guide at website/docs/user-guide/messaging/
whatsapp-cloud.md, sidebar entry, environment-variables reference,
ADDING_A_PLATFORM.md updated with the optional interactive-UX
contract for future adapter authors.

Tests: 100 dedicated tests for the adapter, 32 for the setup wizard,
20 for the Nous subscription STT wiring, plus regression coverage
across display_config, prompt_builder, and the cron scheduler.

Known limitations (deferred until clear demand signal):
- Group chats — use the Baileys bridge if you need them.
- Message templates for 24-hour-window outside-conversation sends —
  reactive chat is unaffected; cron / delegate_task with gaps > 24h
  will fail with a clear error. The agent's system prompt warns the
  model about this so it knows to mention it when scheduling delayed
  messages.

agent/prompt_builder.py

@@ -428,6 +428,23 @@ PLATFORM_HINTS = {
         "files arrive as downloadable documents. You can also include image "
         "URLs in markdown format ![alt](url) and they will be sent as photos."
     ),
+    "whatsapp_cloud": (
+        "You are on a text messaging communication platform, WhatsApp "
+        "(via Meta's official Business Cloud API). Standard markdown "
+        "(**bold**, ~~strike~~, # headers, [links](url)) is auto-converted "
+        "to WhatsApp's native syntax (*bold*, ~strike~, etc.) — feel free "
+        "to write in markdown. Tables are NOT supported — prefer bullet "
+        "lists or labeled key:value pairs. "
+        "You can send media files natively: include MEDIA:/absolute/path/to/file "
+        "in your response. Images (.jpg, .png) become photo attachments, "
+        "videos (.mp4) play inline, audio (.mp3, .ogg) sends as voice/audio "
+        "messages, other files arrive as documents. Image URLs in markdown "
+        "format ![alt](url) also work. "
+        "IMPORTANT: this platform has a 24-hour conversation window — if the "
+        "user hasn't messaged in 24h, free-form replies are refused by Meta "
+        "(error 131047). This rarely matters for live chat, but is worth "
+        "knowing if you're scheduling a delayed message."
+    ),
     "telegram": (
         "You are on a text messaging communication platform, Telegram. "
         "Standard markdown is automatically converted to Telegram format. "
@@ -1279,13 +1296,13 @@ def build_nous_subscription_prompt(valid_tool_names: "set[str] | None" = None) -
 
     lines = [
         "# Nous Subscription",
-        "Nous subscription includes managed web tools (Firecrawl), image generation (FAL), OpenAI TTS, and browser automation (Browser Use) by default. Modal execution is optional.",
+        "Nous subscription includes managed web tools (Firecrawl), image generation (FAL), OpenAI TTS, OpenAI Whisper STT, and browser automation (Browser Use) by default. Modal execution is optional.",
         "Current capability status:",
     ]
     lines.extend(_status_line(feature) for feature in features.items())
     lines.extend(
         [
-            "When a Nous-managed feature is active, do not ask the user for Firecrawl, FAL, OpenAI TTS, or Browser-Use API keys.",
+            "When a Nous-managed feature is active, do not ask the user for Firecrawl, FAL, OpenAI TTS, OpenAI Whisper, or Browser-Use API keys.",
             "If the user is not subscribed and asks for a capability that Nous subscription would unlock or simplify, suggest Nous subscription as one option alongside direct setup or local alternatives.",
             "Do not mention subscription unless the user asks about it or it directly solves the current missing capability.",
             "Useful commands: hermes setup, hermes setup tools, hermes setup terminal, hermes status.",