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.

tests/cron/test_scheduler.py

@@ -2510,3 +2510,26 @@ class TestSendMediaTimeoutCancelsFuture:
         # 2. Second file still got dispatched — one timeout doesn't abort the batch
         adapter.send_video.assert_called_once()
         assert adapter.send_video.call_args[1]["video_path"] == "/tmp/fast.mp4"
+
+
+class TestHomeTargetEnvVarRegistry:
+    """Regression: ``_HOME_TARGET_ENV_VARS`` must include every gateway
+    platform that supports cron-driven outbound delivery. Missing an
+    entry means ``hermes cron create --deliver=<platform>`` silently
+    fails to route through the platform's home channel."""
+
+    def test_whatsapp_cloud_registered(self):
+        """``deliver=whatsapp_cloud`` routes through
+        WHATSAPP_CLOUD_HOME_CHANNEL — added alongside the existing
+        ``whatsapp`` Baileys entry."""
+        from cron.scheduler import _HOME_TARGET_ENV_VARS
+
+        assert "whatsapp_cloud" in _HOME_TARGET_ENV_VARS
+        assert _HOME_TARGET_ENV_VARS["whatsapp_cloud"] == "WHATSAPP_CLOUD_HOME_CHANNEL"
+
+    def test_baileys_whatsapp_still_registered(self):
+        """Sanity guard: the Cloud addition didn't disturb Baileys
+        whatsapp routing."""
+        from cron.scheduler import _HOME_TARGET_ENV_VARS
+
+        assert _HOME_TARGET_ENV_VARS.get("whatsapp") == "WHATSAPP_HOME_CHANNEL"