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/gateway/test_display_config.py

@@ -206,9 +206,23 @@ class TestPlatformDefaults:
         """Signal, BlueBubbles, etc. default to 'off' tool progress."""
         from gateway.display_config import resolve_display_setting
 
-        for plat in ("signal", "bluebubbles", "weixin", "wecom", "dingtalk"):
+        for plat in ("signal", "bluebubbles", "weixin", "wecom", "dingtalk", "whatsapp_cloud"):
             assert resolve_display_setting({}, plat, "tool_progress") == "off", plat
 
+    def test_whatsapp_cloud_locked_to_low_tier_until_edit_message_lands(self):
+        """Regression guard: ``whatsapp_cloud`` must stay TIER_LOW until the
+        adapter implements edit_message. Without an edit endpoint, raising
+        the tier to MEDIUM would spam separate WhatsApp messages for every
+        tool-progress update, which is the exact failure mode this entry
+        exists to avoid.
+
+        When/if Cloud's edit_message lands, update _PLATFORM_DEFAULTS to
+        TIER_MEDIUM and update this test to assert ``"new"`` accordingly.
+        """
+        from gateway.display_config import resolve_display_setting
+        assert resolve_display_setting({}, "whatsapp_cloud", "tool_progress") == "off"
+        assert resolve_display_setting({}, "whatsapp_cloud", "streaming") is False
+
     def test_minimal_tier_platforms(self):
         """Email, SMS, webhook default to 'off' tool progress."""
         from gateway.display_config import resolve_display_setting