refactor(ntfy): convert built-in adapter to platform plugin

ntfy now ships as a self-contained plugin under plugins/platforms/ntfy/
instead of editing 8 core files (gateway/config.py Platform enum,
gateway/run.py factory + auth maps, cron/scheduler.py, toolsets.py,
hermes_cli/status.py, agent/prompt_builder.py, gateway/channel_directory.py,
tools/send_message_tool.py).

All routing goes through gateway/platform_registry via register_platform():
- adapter_factory, check_fn, validate_config, is_connected
- env_enablement_fn seeds PlatformConfig.extra from NTFY_* env vars so
  gateway status reflects env-only setups without instantiating httpx
- standalone_sender_fn handles deliver=ntfy cron jobs when cron runs
  out-of-process from the gateway
- allowed_users_env / allow_all_env hook into _is_user_authorized
- cron_deliver_env_var=NTFY_HOME_CHANNEL for cron home routing
- platform_hint surfaces in the system prompt
- pii_safe=True (topic names are the only identifier; no PII to redact)

Tests moved to tests/gateway/test_ntfy_plugin.py using _plugin_adapter_loader
so the module lives under plugin_adapter_ntfy in sys.modules and cannot
collide with sibling plugin-adapter tests on the same xdist worker. The
core-file grep tests (Platform.NTFY in source, hermes-ntfy in toolsets,
etc.) are replaced with plugin-shape tests covering register() metadata,
env_enablement_fn output, and standalone_sender_fn behavior.

68 tests pass under scripts/run_tests.sh.

tools/send_message_tool.py

@@ -777,8 +777,6 @@ async def _send_to_platform(platform, pconfig, chat_id, message, thread_id=None,
             result = await _send_bluebubbles(pconfig.extra, chat_id, chunk)
         elif platform == Platform.QQBOT:
             result = await _send_qqbot(pconfig, chat_id, chunk)
-        elif platform == Platform.NTFY:
-            result = await _send_ntfy(pconfig, chat_id, chunk)
         elif platform == Platform.YUANBAO:
             result = await _send_yuanbao(chat_id, chunk)
         else:
@@ -1772,28 +1770,6 @@ async def _send_qqbot(pconfig, chat_id, message):
         return _error(f"QQBot send failed: {e}")
 
 
-async def _send_ntfy(pconfig, chat_id, message):
-    """Send a message via ntfy HTTP POST."""
-    try:
-        extra = pconfig.extra or {}
-        server = extra.get("server") or os.getenv("NTFY_SERVER_URL", "https://ntfy.sh").rstrip("/")
-        topic = chat_id or extra.get("topic") or os.getenv("NTFY_TOPIC", "")
-        token = extra.get("token") or os.getenv("NTFY_TOKEN", "")
-        if not topic:
-            return _error("ntfy topic not configured.")
-        import httpx
-        headers = {"Content-Type": "text/plain; charset=utf-8"}
-        if token:
-            headers["Authorization"] = f"Bearer {token}"
-        url = f"{server}/{topic}"
-        async with httpx.AsyncClient(timeout=15.0) as client:
-            resp = await client.post(url, content=message.encode("utf-8"), headers=headers)
-            resp.raise_for_status()
-        return {"success": True, "platform": "ntfy", "chat_id": topic}
-    except Exception as e:
-        return _error(f"ntfy send failed: {e}")
-
-
 async def _send_yuanbao(chat_id, message, media_files=None):
     """Send via Yuanbao using the running gateway adapter's WebSocket connection.