feat(webhook): direct delivery mode for zero-LLM push notifications (#12473)

External services can now push plain-text notifications to a user's chat
via the webhook adapter without invoking the agent. Set deliver_only=true
on a route and the rendered prompt template becomes the literal message
body — dispatched directly to the configured target (Telegram, Discord,
Slack, GitHub PR comment, etc.).

Reuses all existing webhook infrastructure: HMAC-SHA256 signature
validation, per-route rate limiting, idempotency cache, body-size limits,
template rendering with dot-notation, home-channel fallback. No new HTTP
server, no new auth scheme, no new port.

Use cases: Supabase/Firebase webhooks → user notifications, monitoring
alert forwarding, inter-agent pings, background job completion alerts.

Changes:
- gateway/platforms/webhook.py: new _direct_deliver() helper + early
  dispatch branch in _handle_webhook when deliver_only=true. Startup
  validation rejects deliver_only with deliver=log.
- hermes_cli/main.py + hermes_cli/webhook.go: --deliver-only flag on
  subscribe; list/show output marks direct-delivery routes.
- website/docs/user-guide/messaging/webhooks.md: new Direct Delivery
  Mode section with config example, CLI example, response codes.
- skills/devops/webhook-subscriptions/SKILL.md: document --deliver-only
  with use cases (bumped to v1.1.0).
- tests/gateway/test_webhook_deliver_only.py: 14 new tests covering
  agent bypass, template rendering, status codes, HMAC still enforced,
  idempotency still applies, rate limit still applies, startup
  validation, and direct-deliver dispatch.

Validation: 78 webhook tests pass (64 existing + 14 new). E2E verified
with real aiohttp server + real urllib POST — agent not invoked, target
adapter.send() called with rendered template, duplicate delivery_id
suppressed.

Closes the gap identified in PR #12117 (thanks to @H1an1 / Antenna team)
without adding a second HTTP ingress server.

gateway/platforms/webhook.py

@@ -13,6 +13,10 @@ Each route defines:
   - skills: optional list of skills to load for the agent
   - deliver: where to send the response (github_comment, telegram, etc.)
   - deliver_extra: additional delivery config (repo, pr_number, chat_id)
+  - deliver_only: if true, skip the agent — the rendered prompt IS the
+    message that gets delivered.  Use for external push notifications
+    (Supabase, monitoring alerts, inter-agent pings) where zero LLM cost
+    and sub-second delivery matter more than agent reasoning.
 
 Security:
   - HMAC secret is required per route (validated at startup)
@@ -122,6 +126,19 @@ class WebhookAdapter(BasePlatformAdapter):
                     f"For testing without auth, set secret to '{_INSECURE_NO_AUTH}'."
                 )
 
+            # deliver_only routes bypass the agent — the POST body becomes a
+            # direct push notification via the configured delivery target.
+            # Validate up-front so misconfiguration surfaces at startup rather
+            # than on the first webhook POST.
+            if route.get("deliver_only"):
+                deliver = route.get("deliver", "log")
+                if not deliver or deliver == "log":
+                    raise ValueError(
+                        f"[webhook] Route '{name}' has deliver_only=true but "
+                        f"deliver is '{deliver}'. Direct delivery requires a "
+                        f"real target (telegram, discord, slack, github_comment, etc.)."
+                    )
+
         app = web.Application()
         app.router.add_get("/health", self._handle_health)
         app.router.add_post("/webhooks/{route_name}", self._handle_webhook)
@@ -419,6 +436,64 @@ class WebhookAdapter(BasePlatformAdapter):
             )
         self._seen_deliveries[delivery_id] = now
 
+        # ── Direct delivery mode (deliver_only) ─────────────────
+        # Skip the agent entirely — the rendered prompt IS the message we
+        # deliver.  Use case: external services (Supabase, monitoring,
+        # cron jobs, other agents) that need to push a plain notification
+        # to a user's chat with zero LLM cost.  Reuses the same HMAC auth,
+        # rate limiting, idempotency, and template rendering as agent mode.
+        if route_config.get("deliver_only"):
+            delivery = {
+                "deliver": route_config.get("deliver", "log"),
+                "deliver_extra": self._render_delivery_extra(
+                    route_config.get("deliver_extra", {}), payload
+                ),
+                "payload": payload,
+            }
+            logger.info(
+                "[webhook] direct-deliver event=%s route=%s target=%s msg_len=%d delivery=%s",
+                event_type,
+                route_name,
+                delivery["deliver"],
+                len(prompt),
+                delivery_id,
+            )
+            try:
+                result = await self._direct_deliver(prompt, delivery)
+            except Exception:
+                logger.exception(
+                    "[webhook] direct-deliver failed route=%s delivery=%s",
+                    route_name,
+                    delivery_id,
+                )
+                return web.json_response(
+                    {"status": "error", "error": "Delivery failed", "delivery_id": delivery_id},
+                    status=502,
+                )
+
+            if result.success:
+                return web.json_response(
+                    {
+                        "status": "delivered",
+                        "route": route_name,
+                        "target": delivery["deliver"],
+                        "delivery_id": delivery_id,
+                    },
+                    status=200,
+                )
+            # Delivery attempted but target rejected it — surface as 502
+            # with a generic error (don't leak adapter-level detail).
+            logger.warning(
+                "[webhook] direct-deliver target rejected route=%s target=%s error=%s",
+                route_name,
+                delivery["deliver"],
+                result.error,
+            )
+            return web.json_response(
+                {"status": "error", "error": "Delivery failed", "delivery_id": delivery_id},
+                status=502,
+            )
+
         # Use delivery_id in session key so concurrent webhooks on the
         # same route get independent agent runs (not queued/interrupted).
         session_chat_id = f"webhook:{route_name}:{delivery_id}"
@@ -572,6 +647,34 @@ class WebhookAdapter(BasePlatformAdapter):
     # Response delivery
     # ------------------------------------------------------------------
 
+    async def _direct_deliver(
+        self, content: str, delivery: dict
+    ) -> SendResult:
+        """Deliver *content* directly without invoking the agent.
+
+        Used by ``deliver_only`` routes: the rendered template becomes the
+        literal message body, and we dispatch to the same delivery helpers
+        that the agent-mode ``send()`` flow uses.  All target types that
+        work in agent mode work here — Telegram, Discord, Slack, GitHub
+        PR comments, etc.
+        """
+        deliver_type = delivery.get("deliver", "log")
+
+        if deliver_type == "log":
+            # Shouldn't reach here — startup validation rejects deliver_only
+            # with deliver=log — but guard defensively.
+            logger.info("[webhook] direct-deliver log-only: %s", content[:200])
+            return SendResult(success=True)
+
+        if deliver_type == "github_comment":
+            return await self._deliver_github_comment(content, delivery)
+
+        # Fall through to the cross-platform dispatcher, which validates the
+        # target name and routes via the gateway runner.
+        return await self._deliver_cross_platform(
+            deliver_type, content, delivery
+        )
+
     async def _deliver_github_comment(
         self, content: str, delivery: dict
     ) -> SendResult: