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.

hermes_cli/status.py

@@ -423,7 +423,6 @@ def show_status(args):
         "BlueBubbles": ("BLUEBUBBLES_SERVER_URL", "BLUEBUBBLES_HOME_CHANNEL"),
         "QQBot": ("QQ_APP_ID", "QQ_HOME_CHANNEL"),
         "Yuanbao": ("YUANBAO_APP_ID", "YUANBAO_HOME_CHANNEL"),
-        "ntfy": ("NTFY_TOPIC", "NTFY_HOME_CHANNEL"),
     }
 
     for name, (token_var, home_var) in platforms.items():